Idont understand what they refer to when they say what 128 is 74% of what number.

==>  128  is  100%  of  128 .

==>  128  is  200%  of  64 .

==>  128  is  50%  of  256.

==>  128  is  10%  of  1,280 .

Now, what number is it  74%  of ?

Here's how to find it:

Call the mystery number ' M ' .

The question says that  128  is  74%  of ' M ' .

You know that  74%  means  0.74 .
And you know that in math, "of" usually means "times".
So you can write the question as
                                                           128 = 0.74 times M

                                                           128 = 0.74 M

Divide each side by  0.74 :            128 / 0.74 = M

                                                           172.973  =  M

There's the mystery number.  128  is (about)  74%  of  172.973 .

D.