A jazz concert brought in $124,000 on the sale of 7,900 tickets. If tickets sold for $10 and $20 each, how many of each type of tickets were sold?

This problem gives you two unknowns (Two different types of tickets). That means you will need at least two equations.
Lets say the $10.00 ticket is named "x" and the $20.00 ticket is named "y". Your two equations would be as follows...

$10.00x + $20.00y = $124,000 eqn 1

x + y = 7,900 tickets eqn 2

solve for either x or y in one of the equations. I use eqn 2 because it looks more simple.

x = 7900 - y eqn 3

now "plug" eqn 3 into eqn 1

$10.00 (7900 - y) + 20.00y = 124000 eqn 4

if you factor this out you are left with

79000 - 10y + 20y = 124000 eqn 5

From here just get y by itself. Once you have y you can plug it into eqn 2 and you will have your answer. I hope I set it up enough for you to figure out. Give it a shot and let me know if you are still stuck.