Working with computers I mentally use a hybrid of BCD and binary arithmetic. e.g. for the 17.5%, I mentally move the decimal point to the left, drop the cents,

Working with computers I mentally use a hybrid of BCD and binary arithmetic. e.g. for the 17.5%, I mentally move the decimal point to the left, drop the cents, add half and then half of that, i.e. 10% + 5% + 2.5%