if 3 : 7 : : x : 21 , then find the value of x
explanation : treat 3 : 7 as 3 / 7 and x : 21 as x / 21 , treat : : as = so we get 3 / 7 = x / 21 = > 7 x = 63 = > x = 9 option e
divide(add(multiply(7, 3), 7), 21)