So, there are 3 Apples. Oranges = 2 * 3 Apples = 6 Oranges. The sum of the number of Oranges and Apples represent half of the Cherries. Mathematically speaking, o + a = 1/2 c <=> 3 + 6 = 1/2c. => c=18. So there are 18 Cherries.
You just start from 6 and 0 and keep on going like 7 and 1, 8 and 2, 9 and 3 etc. When I reached 18 and 12, the sum was 30, and it was 6 numbers apart.
