Physics question belows?

Question

A train consists of 50 cars, each of which has a mass of 6.0 103 kg. The train has an acceleration of 1.5 10-1 m/s2. Ignore friction and determine the tension in the coupling at the following places.
(a) between the 30th and 31st cars

(b) between the 49th and 50th cars

Answer 1

(a) 20 cars x 6.0103 kg x 1.510 = 181.5 Newtons
(b) 1 car x 6.0103 kg x 1.510 = 9.1 Newtons

Answer 2

First look at the last car (50). It is accelerating at rate a = 1.5*10^-1m/sec^2. The only thing pulling that car is the coupling between the 49th and 50th cars, so the tension in that coupling must be mass of car times a, or 6*10^3 kg * 1.5*10^-1 m/sec^2 = 9*10^2N; For each car further up, the tension increases by the m*a of each additonal car being pulled; the formula for car n would then be T = (51 - n)*m*a; for car 31, it would be 20*m*a or 1.8*10^4N