there are 88 keys on a piano. some keys are black and some keys are white.

Question

if x represents the number of white keys, which expression below represents the number of black keys

Answer 1

I don't think you listed your options, but if we know that there is a total of 88 of anything, and there are "x" of some of these things, what remains = 88 minus x.

 

Imagine 88 marbles.

Imagine a certain number of them, which we haven't counted, are red.

If we take away all of the red ones, how many marbles do we have?

 

We have the total number of marbles minus the red ones, which we shall denote as x.

 

The way to express this numerically is 88 - x, x being a symbol for the unknown number of red marbles. You can use the same logic to apply to your problem about the number of keys on a piano, except there is a total of 88 keys, with some being black and some white.

Answer 2

The wording of this question suggests that this is a multiple choice question. Therefore you should have included those choices.

 

Without that, I will provide my answer: # black keys = 88 - x.