Answer :
L=hours Lincoln worked
H=hours Hunter worked
Two unknowns , two equations
30L+20H=350
L+H=15
Solve
L=15-H
30(15-H)+20H=350
450-30H+20H=350
-10H=-100
10H=100
H=10
L=15-10=5
Lincoln worked 10 hours
Hunter worked 5 hours
Check
30*5+20*10=350
150+200=350
350=350
H=hours Hunter worked
Two unknowns , two equations
30L+20H=350
L+H=15
Solve
L=15-H
30(15-H)+20H=350
450-30H+20H=350
-10H=-100
10H=100
H=10
L=15-10=5
Lincoln worked 10 hours
Hunter worked 5 hours
Check
30*5+20*10=350
150+200=350
350=350