A group of visitors paid a total of $380 for entrance tickets to a zoo. The entrance ticket for a childs cost $4. The entrance ticket for an adult cost twiceas much. If there were 10 more adults than children, how many adults were there.

4 c + 8 a = 380

c = a - 10
====================
substitute (a-10) for c

4 (a -10) + 8 a = 380

12 a - 40 = 380

12 a = 420

a = 35