how many numbers from 10 to 31 are exactly divisible by 3 ?
"12 , 15 , 18 , 21 , 24 , 27 , 30 . 7 numbers . 10 / 3 = 3 and 31 / 3 = 10 = = > 10 - 3 = 7 . therefore 7 digits d )"
add(divide(subtract(multiply(floor(divide(31, 3)), 3), multiply(add(floor(divide(10, 3)), const_1), 3)), 3), const_1)