If the average of six numbers is 18 and the average of three of the numbers is 15, then what is the sum of the remaining three numbers?
21
53
45
63

If the average of six numbers is 18, there sum may be calculated by multiplying this average with 6 which gives us, 108. Three of these numbers have the average of 15. By the same principle, the sum of these numbers is 45. The sum of the remaining numbers in the set may be obtained by subtracting the sum of the three from the total, 108 - 45 which is equal to 63. Thus, the answer is 63. 

25 is obviously the correct answer