what is the average of first 21 multiples of 6 ?
"required average = 6 ( 1 + 2 + . . . . + 21 ) / 21 ( 6 / 21 ) x ( ( 21 x 22 ) / 2 ) ( because sum of first 21 natural numbers ) = 66 a"
multiply(divide(divide(multiply(21, add(21, const_1)), const_2), 21), 6)