Question

Use mental math to solve the equation 1.60=0.04+s

Answer 1

You have to use the inverse operation:
 
If 1.60 = 0.04 + s,  s = 1.60 - 00.4

s = 1.56

How to prove:

1.60 = 1.56 + 0.04

Hope it helped!

Answer 2

Well using mental math, subtract .04 from 1.6 to get 1.56, which is s