find the average of first 20 multiples of 7 ?
"required average = 7 ( 1 + 2 + 3 + … … . + 20 ) / 20  ( 7 * 20 * 21 ) / ( 20 * 2 )  ( 147 / 2 ) = 73.5 . answer a 73.5"
divide(add(add(add(20, const_1), add(add(20, const_1), const_2)), add(subtract(7, 20), subtract(7, const_2))), 20)