Question 636
Let's do this logically.
What if half the coins were dimes? You'd have 12 dimes and 12 nickels, a total of $1.80. Too Low.
The number of dimes can't be odd, because if that were so, the number of nickels would be odd also, giving us an answer ending in 5.
Let's try 14 dimes. 14 dimes and 10 nickels equals $1.90. Hmm, that's odd, if the number of dimes goes up by two, the total goes up by 10 cents. To go up thirty more cents to our goal, we add six more dimes. 20 dimes and 4 nickels equals $2.20.

Use that method for all problems like that, guess and check.