lets take the certain number and call it 'x'
take 2.5 of the number = 2.5x
when we add 1.75 - 2.5x + 1.75
and divide by 0.8 - (2.5x+ 1.75)/0.8
the answer is 37.5
lets construct an equation with this information
(2.5x + 1.75)/0.8 = 37.5
2.5x + 1.75 = 37.5* 0.8
2.5x = 30 - 1.75
2.5x = 28.25
x = 28.25/2.5
x = 11.3