B. The force of X on Y is is approximately 13 N [right].
The action-reaction forces that the blocks exert on each other is calculated as follows;
Net force on the blocks = 40 N - (8 N + 4 N) = 28 N
Total mass of the blocks = 10 kg + 5 kg = 15 kg
a = F(net)/M
a = 28 N / 15 kg
a = 1.867 m/s²
40 N - 8N - T = 10 x 1.867
T = 13.33 N
40 N - 4N - T = 5 x 1.867
T = 26.67 N
Thus, the force of X on Y is is approximately 13 N [right].
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