diff --git a/src/main/java/com/serenitydojo/calculator/Calculator.java b/src/main/java/com/serenitydojo/calculator/Calculator.java
index 8e88bf0..47c0dc0 100644
--- a/src/main/java/com/serenitydojo/calculator/Calculator.java
+++ b/src/main/java/com/serenitydojo/calculator/Calculator.java
@@ -2,6 +2,31 @@
 
 public class Calculator {
     public int evaluate(String expression) {
-        return 0;
+
+        String operators[] = expression.replaceAll("\\s", "").split("[0-9]+");
+        String operands[] = expression.replaceAll("\\s", "").split("[*/+-]");
+
+        int runningTotal = Integer.parseInt(operands[0]);
+
+        for(int i=1; i<operands.length; i++) {
+            if(operators[i].equals("+")) {
+                runningTotal += Integer.parseInt(operands[i]);
+            }
+            else if(operators[i].equals("-")) {
+                runningTotal -= Integer.parseInt(operands[i]);
+            }
+            else if(operators[i].equals("*")) {
+                runningTotal *= Integer.parseInt(operands[i]);
+            }
+            else if(operators[i].equals("/")) {
+                runningTotal /= Integer.parseInt(operands[i]);
+            }
+            else
+            {
+                throw new IllegalMathsOperatorException("Unknown operator ");
+            }
+        }
+        return runningTotal;
     }
+
 }
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..92cea87
--- /dev/null
+++ b/src/main/java/com/serenitydojo/calculator/IllegalMathsOperatorException.java
@@ -0,0 +1,7 @@
+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..a23283d 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 +12,38 @@ 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() throws IllegalMathsOperatorException {
+        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);
+    }
 }