He saved 2500 in April.
His monthly salary is twice that saved in April = 2 * 2500 = 5000
Let the amount saved in May = m
Amount spent in May = 4200+ m
For the Month of May:
Monthly Salary - Amount Spent = Amount Saved.
5000 - (4200 + m) = m
5000 -4200 - m = m
800 = m + m
800 = 2m
800/2 = m
400 = m
Amount saved in May = 400 dollars.