Answer:
For those who can't see the answer 
Saying that multiple choice questions are m, the equation is: 
18m+19=15m+31 
subtract 19 from both sides to get: 
18m=15m+12 
subtract 15m from both sides to get: 
3m=12 
divide by three to get the answer: 
m=4 
Each multiple choice question is worth 4 points
Explanation: