You spend 1/2 of your allowance on Monday. On Tuesday, you spend 1/2 of the amount you have left. On Wednesday, you spend 1/2 of what remains. What fraction of your allowance do you have left?
Given that the allowance is variable. It varies in a week. Let X = be your allowance. Monday to Wednesday, you already incurred Monday = X / 2 Tuesday = Monday / 2 Wednesday = Tuesday / 2 The money left you have will be Money left = X - (Monday + Tuesday + Wednesday)