Question

Fifteen students were surveyed to see whether they liked the color red or blue. There were three more students who preferred blue over red. Write and solve a system of equations to find how many students preferred blue and how many preferred red.

Answer 1

r=students who like color red
b=students who like color blue

r+b=15
b=r+3

then, replace the b in the first equation with the b in the second:
r+(r+3)=15
2r=12
r=6

6 students like red

r+b=15
6+b=15

b=9

6 students like red 
9 students like blue