diff --git a/src/main/java/tdd/setup/Calculator.java b/src/main/java/tdd/setup/Calculator.java
index 171f6d6..f1e3a3e 100644
--- a/src/main/java/tdd/setup/Calculator.java
+++ b/src/main/java/tdd/setup/Calculator.java
@@ -12,11 +12,17 @@ public class Calculator {
     public String readScreen() {
         return screen;
     }
+
     public void pressDigitKey(int digit) {
         if(digit > 9 || digit < 0) throw new IllegalArgumentException();
 
         if(latestOperation.isEmpty()) {
-            screen = screen + digit;
+            if (screen == "0") {
+                screen = Integer.toString(digit);
+            }
+            else {
+                screen = screen + digit;
+            }
         } else {
             latestValue = Double.parseDouble(screen);
             screen = Integer.toString(digit);
@@ -30,7 +36,13 @@ public void pressClearKey() {
     }
 
     public void pressOperationKey(String operation)  {
-        latestOperation = operation;
+        if (latestOperation == "") {
+            latestOperation = operation;
+        }
+        else {
+            pressEquals();
+            latestOperation = operation;
+        }
     }
 
     public void pressDotKey() {
diff --git a/src/test/java/tdd/setup/CalculatorTest.java b/src/test/java/tdd/setup/CalculatorTest.java
index 3bf16e9..28bdca2 100644
--- a/src/test/java/tdd/setup/CalculatorTest.java
+++ b/src/test/java/tdd/setup/CalculatorTest.java
@@ -16,4 +16,38 @@ void calculatorCanDoTwoPlusTwo() {
         calc.pressEquals();
         assertEquals("4", calc.readScreen());
     }
+
+    @Test
+    @DisplayName("should display result after subtracting two positive numbers")
+    void calculatorCanSubtract() {
+        Calculator calc = new Calculator();
+        calc.pressDigitKey(5);
+        calc.pressOperationKey("-");
+        calc.pressDigitKey(6);
+        calc.pressEquals();
+        assertEquals("-1", calc.readScreen());
+    }
+
+    @Test
+    @DisplayName("should display result after multiplying two numbers and add a third number")
+    void calculatorCanMultiply() {
+        Calculator calc = new Calculator();
+        calc.pressDigitKey(3);
+        calc.pressOperationKey("x");
+        calc.pressDigitKey(6);
+        calc.pressOperationKey("+");
+        calc.pressDigitKey(2);
+        calc.pressEquals();
+        assertEquals("20", calc.readScreen());
+    }
+
+    @Test
+    @DisplayName("should display number with dot")
+    void calculatorCanUseDot() {
+        Calculator calc = new Calculator();
+        calc.pressDigitKey(2);
+        calc.pressDotKey();
+        calc.pressDigitKey(5);
+        assertEquals("2.5", calc.readScreen());
+    }
 }