a+b+ab=35 => ab+a+b+1=35+1=36 => (a+1)(b+1)=36. a and b have to be positive whole numbers, so you need two numbers x and y so that x*y=36, x and y distinct and neither can be 1. There are multiple options.

One way is to exhaust the options for b and find a corresponding answer for a. A hint with this method is that if you plug (5,5) into the expression a+b+ab you get 35. However this isn’t a solution to my question as a is not strictly bigger than b. However, this does mean that b<5, as if b>5, a>5 which means the expression is bigger than 35, and using (5,5) gives 35 as shown before.

Another is to rearrange the equation into a form where you can factorise it. If you want help with this second way feel free to ask.

public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser)

float (marimba volume) = (float) Math.round(144 * Math.cos(progress * Math.PI / 144)) / 144;
float (harp volume) = (float) Math.round(144 * Math.sin(progress * Math.PI / 144)) / 144;