The assets must equal liabilities, otherwise there is an error in your balance sheet.

Twenty euros cover charge? You're joking me!

If it's an owie that has the child upset, check him from head to toe for injuries.

Given an integer X written in base 10 (as is usual), compute the sum of the digits of X: call the result the digit sum of X. If the digit sum is greater than 9, we form the digit sum again. Continue in this way to obtain the iterated digit sum which is at most 9.