A force of 400 N gives a mass (m1) an acceleration of 2 m/s^2. The same force gives another mass (m2) an acceleration of 13 m/s^2. What acceleration will this same force give if m1 and m2 are fastened together?

F=M1*a1.

M1 = F/a1 = 400/2 = 200 kg.

M2 = F/a2 = 400/13 = 30.8 kg.

a = F/(M1+M2).