Click on play and then on the level number you are allowed to choose.
Your goal is to select the math operation (plus, minus, multiply or divide by). A number will appear above your head and another will appear in the skull on top of the screen. The orc coming your way will have a third number. You must use the correct math operation to create the correct math formula. For example, if you have 10 and the skull has 2 and the orc has 8, you must select minus. So 10 - 8 = 2. If you have 25 and the skull has 32 and the orc has 7, you'll need to select + so 25 + 7 = 32.
You must complete these operations before the orc gets to you. All you need to do is use your mouse button or your finger to select the right math operation.