First, let's define variables:
A = adults
C = children
Okay, now that's done, time to set up the two equations required to solve:
10a + 7c = 6549
a + c = 783
One way you can solve this is to set up the second equation to isolate a variable. For example,
a = -c + 783
Using this, we can plug it into the first equation and solve for c.
10(-c+783) + 7c = 6549
-3c = -1281
c = 427
Now, use the value of 427and plug it in either equation to solve for a.
a + 427 = 783
a = 356
356 adult tickets were sold, and 427 children tickets were sold.