agreed with pepeprepa on this... just do some trial and error.
But there is one thing you could do which would narrow your choices down to a couple to test:
Looking at 10, 12 and 18.. and knowing that we need 14 as an average. You can basically see that if there were no $12s you'd need an equal number of 10s and 18s to get an average of 14 (since (10+18)/2 = 14). From there, if you were to substitute some of the 10s for 12s, you'll see that you'll need a smaller number of 18s to get that same 14 average.
So what does this mean? The number of 10s and 12s should, together, be more than the number of 18s.
Another way to look at it is that if you were combine only the 10s and 12s, the lowest the average could be is 10 and the highest 12. If it was 10, there would be a 1:1 ration with the 18s.. and if the average was 12, there would be a 2:1 ration with 18s. Of course we're looking for something in between.
In any case, this can eliminate choices A, C, and E... So you'd only need to test B and D.
Of course, if you were to realize this, you'd only really save 20-30 seconds.. so it's not the end of the world if you don't.
