If 5 A × A = 399, then the value of A is

According to the question

A x A = 9

⇒ The one’s place in the product of A and A is 9

∴ A is the number whose multiplication with itself gives a two digit number having 9 in it’s one’s place.

Hence A = 7 because A x A = 49 and one’s place of 49 is 9

∴ Option (c) is correct.