Question

Why are the MR and AR of a perfectly competitive firm equal ?

Answer 1

Because price of the good remains same at all levels of output due to the existence of a large number of firms and the firms produce and sell homogeneous product. 

Output	Price	TR	AR
1	10	10	10
2	10	20	10
3	10	30	10
4	10	40	10
5	10	50	10