Alice likes to go to Main Event to play laser tag and climb the rock wall each Saturday. Instead of purchasing the play pass, Alice pays for each activity separately. This week Alice played laser tag twice and climbed the rock wall 3 times, spending $31.
Last week Alice played laser tag 4 times and climbed the rock wall once, spending $27.

Using the information provided, how much does Main Event charge, in dollars, to climb the rock wall?