A constant force acts on an object of mass 2 kg for 10s and increases its velocity from 5ms-1 to 10ms-1. Find the magnitude of applied force. If this force was applied for a duration of 15s, what would be the velocity of the object ?

Use F=ma, where a = \frac {\Delta v} t . The second part simply uses the same value of a, and you can calculate the change in velocity using the new value for time.