the average of 6 numbers is 30 . if the average of first 4 is 25 and that of last 3 is 35 , the fourth number is :
let the six numbers be , a , b , c , d , e , f . a + b + c + d + e + f = 30 × 6 = 180 - - - - ( 1 ) a + b + c + d = 25 × 4 = 100 - - - - ( 2 ) d + e + f = 35 × 3 = 105 - - - - ( 3 ) add 2 nd and 3 rd equations and subtract 1 st equation from this . d = 25 answer : a
subtract(multiply(3, 35), subtract(multiply(6, 30), multiply(4, 25)))