Answer:
The answer to your question is $9.5
Explanation:
Data
Large pizza = L
Medium pizza = m
Write equation for both situations
Situation 1  5L + 2m = 81.5
Situation 2 4L + 3m = 78.5
Solve the system of equation by elimination
Multiply situation 1 by -3 and situation 2 by 2
 -15L - 6m = -244.5
 8L + 6m = 157
 -7L = -87.5
Solve for L L = -87.5/-7
  L = $12.5 
Substitute L in Situation 1 to find m
 5(12.5) + 2m = 81.5
 62.5 + 2m = 81.5
 2m = 81.5 - 62.5
 2m = 19
 m = 19/2
  m = $ 9.5
The prize of a medium pizza is $9.5