diff --git a/src/main/java/com/serenitydojo/calculator/Calculator.java b/src/main/java/com/serenitydojo/calculator/Calculator.java
index 8e88bf0..88b40ca 100644
--- a/src/main/java/com/serenitydojo/calculator/Calculator.java
+++ b/src/main/java/com/serenitydojo/calculator/Calculator.java
@@ -1,7 +1,40 @@
 package com.serenitydojo.calculator;
 
+import org.apache.commons.lang3.StringUtils;
+
 public class Calculator {
     public int evaluate(String expression) {
-        return 0;
+        if(expression.isEmpty()) {
+            return 0;
+        }
+
+        int calculationResult = 0;
+        String currentOperator = "+";
+
+        String[] characters = expression.split("\\s");
+
+        for(String character : characters) {
+            if (!StringUtils.isNumeric(character)) {
+                currentOperator = character;
+            } else {
+                calculationResult = calculation(calculationResult, currentOperator, character);
+            }
+        }
+        return calculationResult;
+    }
+
+    private int calculation(int calculationResult, String currentOperator, String character) throws IllegalMathsOperatorException {
+        switch(currentOperator) {
+            case "+":
+                return calculationResult + Integer.parseInt(character);
+            case "-":
+                return calculationResult - Integer.parseInt(character);
+            case "*":
+                return calculationResult * Integer.parseInt(character);
+            case "/":
+                return calculationResult / Integer.parseInt(character);
+            default:
+                throw new IllegalMathsOperatorException(currentOperator + " operator is not supported!");
+        }
     }
-}
+}
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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..2a59e68
--- /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);
+    }
+}
\ No newline at end of file
diff --git a/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java b/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java
index eac4b31..b6e8b22 100644
--- a/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java
+++ b/src/test/java/com/serenitydojo/calculator/WhenDoingMaths.java
@@ -13,38 +13,43 @@ 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 shouldDivideNumbers() {
+        assertThat(calculator.evaluate("25 / 5")).isEqualTo(5);
+    }
+}
\ No newline at end of file