what is the average of first 21 multiples of 5 ?
required average = 7 ( 1 + 2 + . . . . + 21 ) / 21 ( 5 / 21 ) x ( ( 21 x 22 ) / 2 ) ( because sum of first 21 natural numbers ) = 55 c
multiply(divide(divide(multiply(21, add(21, const_1)), const_2), 21), 5)