Answer:
t = 0.67 [s]
Step-by-step explanation:
To solve this problem we must use the following kinematics equation.

Vf = final velocity = 20[m/s]
Vi = initial velocity = 10 [m/s]
a = aceleration = 15 [m/s^2]
Now replacing in the equation we have:
20 = 10 + (15*t)
t = (20-10)/15
t = 0.67 [s]