diff --git a/pom.xml b/pom.xml
index f4d0ad5..c24faeb 100644
--- a/pom.xml
+++ b/pom.xml
@@ -23,21 +23,21 @@
       <artifactId>junit</artifactId>
       <version>4.13</version>
       <scope>test</scope>
+    </dependency>
+     <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <version>3.18.0</version>
     </dependency>
     <dependency>
       <groupId>com.google.guava</groupId>
       <artifactId>guava</artifactId>
-      <version>30.0-jre</version>
+      <version>30.1-jre</version>
     </dependency>
     <dependency>
       <groupId>org.apache.commons</groupId>
       <artifactId>commons-text</artifactId>
-      <version>1.8</version>
-    </dependency>
-    <dependency>
-      <groupId>org.assertj</groupId>
-      <artifactId>assertj-core</artifactId>
-      <version>3.18.0</version>
+      <version>1.9</version>
     </dependency>
   </dependencies>
 
diff --git a/src/main/java/com/serenitydojo/calculator/Calculator.java b/src/main/java/com/serenitydojo/calculator/Calculator.java
index 8e88bf0..c37a46b 100644
--- a/src/main/java/com/serenitydojo/calculator/Calculator.java
+++ b/src/main/java/com/serenitydojo/calculator/Calculator.java
@@ -1,7 +1,45 @@
 package com.serenitydojo.calculator;
 
+import com.google.common.base.Splitter;
+
+import java.util.List;
+
+import static org.apache.commons.lang3.StringUtils.isNumeric;
+
 public class Calculator {
     public int evaluate(String expression) {
-        return 0;
+        if (expression.isEmpty()) {
+            return 0;
+        }
+        List<String> tokens = Splitter.on(" ").splitToList(expression);
+
+        int runningTotal = 0;
+        String nextOperator = "+";
+
+        for(String token : tokens) {
+            if (!isNumeric(token)) {
+                nextOperator = token;
+            } else {
+                runningTotal = process(runningTotal, nextOperator, token);
+            }
+        }
+
+        return runningTotal;
+    }
+
+    private int process(int runningTotal, String nextOperator, String token) {
+        switch (nextOperator) {
+            case "+":
+                return runningTotal + Integer.parseInt(token);
+            case "-":
+                return runningTotal - Integer.parseInt(token);
+            case "*":
+                return runningTotal * Integer.parseInt(token);
+            case "/":
+                return runningTotal / Integer.parseInt(token);
+            default:
+                throw new IllegalMathsOperatorException("Unknown operator " + nextOperator);
+        }
     }
+
 }
diff --git a/src/main/java/com/serenitydojo/calculator/IllegalMathsOperatorException.java b/src/main/java/com/serenitydojo/calculator/IllegalMathsOperatorException.java
new file mode 100644
index 0000000..bc31a0a
--- /dev/null
+++ b/src/main/java/com/serenitydojo/calculator/IllegalMathsOperatorException.java
@@ -0,0 +1,8 @@
+package com.serenitydojo.calculator;
+
+public class IllegalMathsOperatorException extends RuntimeException {
+    public IllegalMathsOperatorException(String message) {
+        super(message);
+    }
+
+}
diff --git a/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java b/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java
index eac4b31..eeeadfb 100644
--- a/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java
+++ b/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java
@@ -1,6 +1,5 @@
 package com.serenitydojo.calculator;
 
-import org.apache.commons.lang3.StringUtils;
 import org.junit.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -13,38 +12,53 @@ public void shouldReturnZeroForAnEmptyString() {
         assertThat(calculator.evaluate("")).isEqualTo(0);
     }
 
-//    @Test(expected = IllegalMathsOperatorException.class)
-//    public void shouldReportNonSupportedOperations() {
-//        calculator.evaluate("1 ^ 2");
-//    }
-//
-//    @Test
-//    public void shouldReturnTheValueOfASingleNumber() {
-//        assertThat(calculator.evaluate("1")).isEqualTo(1);
-//    }
-//
-//    @Test
-//    public void shouldAddTwoNumbers() {
-//        assertThat(calculator.evaluate("1 + 1")).isEqualTo(2);
-//    }
-//
-//    @Test
-//    public void shouldAddThreeNumbers() {
-//        assertThat(calculator.evaluate("1 + 2 + 3")).isEqualTo(6);
-//    }
-//
-//    @Test
-//    public void shouldAlsoSubtract() {
-//        assertThat(calculator.evaluate("10 - 6")).isEqualTo(4);
-//    }
-//
-//    @Test
-//    public void shouldAddAndSubtract() {
-//        assertThat(calculator.evaluate("10 + 5 - 6")).isEqualTo(9);
-//    }
-//
-//    @Test
-//    public void shouldMultiplyNumbers() {
-//        assertThat(calculator.evaluate("10 * 5")).isEqualTo(50);
-//    }
+    @Test(expected = IllegalMathsOperatorException.class)
+    public void shouldReportNonSupportedOperations() {
+        calculator.evaluate("1 ^ 2");
+    }
+
+    @Test
+    public void shouldReturnTheValueOfASingleNumber() {
+        assertThat(calculator.evaluate("1")).isEqualTo(1);
+    }
+
+    @Test
+    public void shouldAddTwoNumbers() {
+        assertThat(calculator.evaluate("1 + 1")).isEqualTo(2);
+    }
+
+    @Test
+    public void shouldAddThreeNumbers() {
+        assertThat(calculator.evaluate("1 + 2 + 3")).isEqualTo(6);
+    }
+
+    @Test
+    public void shouldAlsoSubtract() {
+        assertThat(calculator.evaluate("10 - 6")).isEqualTo(4);
+    }
+
+    @Test
+    public void shouldAddAndSubtract() {
+        assertThat(calculator.evaluate("10 + 5 - 6")).isEqualTo(9);
+    }
+
+    @Test
+    public void shouldMultiplyNumbers() {
+        assertThat(calculator.evaluate("10 * 5")).isEqualTo(50);
+    }
+
+    @Test
+    public  void shouldDivideTwoNumbers(){
+        assertThat(calculator.evaluate("10 / 2")).isEqualTo(5);
+    }
+
+    @Test
+    public void shouldDivideAndAddOneNumber(){
+        assertThat(calculator.evaluate("10 / 2 + 2")).isEqualTo(7);
+    }
+
+    @Test
+    public void shouldDivideAndMultiplyOneNumber(){
+        assertThat(calculator.evaluate("10 / 2 * 5")).isEqualTo(25);
+    }
 }