When the expression (7^1)(7^2)(7^3) ... (7^100) is written as an integer, what is the product of the tens digit and the ones digit?
productfor(n, 1, 100, 7^n) mod 10^10 ==6696251249 - these are the last 10 digits.
4 x 9 ==36 - product of tens digit and ones digit.
