The average of six numbers is 30. If the average of first four is 25 and that of last three is 35, the fourth number is :
Select:  A) 25  B) 26  C) 18  D) 19  E) 10
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 2nd and 3rd equations and subtract 1st equation from this.   d = 25   25nswer: 25
<result>A</result>
The average of six numbers is 30. If the average of first four is 25 and that of last three is 35, the fourth number is :