Force = (mass) x (acceleration)
Mass = (force) / (acceleration)
There was 150N of force in one direction and 100N of 'force' in
the other direction. The net force on the object was (150 - 100) = 50N .
Acceleration = (change in speed) / (time for the change)
= (10 m/s) / (5s) = 2 m/s²
Mass = (net force) / (acceleration) = (50 N) / (2 m/s²) = 25 kilograms