what is the tens digit of 6 ^ 21 ?
"the tens digit of 6 in integer power starting from 2 ( 6 ^ 1 has no tens digit ) repeats in a pattern of 5 : { 3 , 1 , 9 , 7 , 5 } : the tens digit of 6 ^ 2 = 36 is 3 . the tens digit of 6 ^ 3 = 216 is 1 . the tens digit of 6 ^ 4 = . . . 96 is 9 . the tens digit of 6 ^ 5 = . . . 76 is 7 . the tens digit of 6 ^ 6 = . . . 56 is 5 . the tens digit of 6 ^ 7 = . . . 36 is 3 again . etc . . . 21 has the form 5 n + 1 , so the tens digit of 6 ^ 21 is 5 . the answer is c ."
floor(divide(reminder(power(6, reminder(21, add(const_4, const_1))), const_100), const_10))