Question: If 3 is the least prime factor of number a and 7 is the least prime factor of number b, then the least prime factor of a + b, is 

Solu:

Let 3 is the least prime factor of a and 7 is the least prime factor of a number b

So, a should be greater than or equal to 3 and b should be greater than or equal to 7

i.e. a >= 3¹ and b >= 7¹

Let a = 3 and b = 7

Now a + b = 3 + 7 = 10

So, 2 is the least prime factor of a + b

Again let a = 3² and b = 7²

Now a + b = 9 + 49 = 58, which is an even number.

So, the least prime factor of a + b is 2