How long will it

Force applied (F) = -10 N (since the force is acting in the opposite direction of motion or opposing the motion)

Mass of body (m) = 2.5kg

Initial velocity of body (u) = 20m/s

Final velocity of body (v) = 0m/s

According to Newton's second law of motion

F = ma

-10 = 2.5×a

-10/2.5 = v-u/t (since a = v-u/t)

-4t = 0 - 20

t = -20/-4

t = 5 sec