∵7n=(10−3)n=10k+(−3)n
7n+3n=10k+(−3)n+3n
10k+{(-3)}^{n}+{(3)}^{n}={\begin{matrix}10k\text{if n=odd} \\ 10k+2.{3}^{n}\text{if n=even}\end{matrix}
Let n=2t;t∈N
∴3n=32t=(10−1)t
=10p+(−1)t
.=10p±1
∴ if n= even then 7n+3n will not be multiply of 10
So if n is odd then only 7n+3n will be multiply of 10
∴n=11,13,15,……….,99
∴ Number of odd two digit numbers =45