Rick deposited $850 to his bank account

Rick deposited $850 to his bank account using $5 and $15 dollar bills only. If Rick came to the bank with 70 bills and left with 10 bills only, how many $15 dollar bills did he deposit?

(A)5.
(B)25.
(C)50.
(D)55.
(E)65.

The best answer is D.
Rick came to the bank with 70 bills and left with 10 and therefore he deposited 60 bills.
Define X as the number of $5 dollar bills that he deposited and so (60 – X) is the number of $15 bills that he deposited.
We can write the following equation: 5X + 15(60 – X) = 850 ? -10X = -50 ? X = 5.
The number of $15 dollar bills is (60 – 5) 55.