Question 97153
T+F=42 OR T=42-F
45*3T+45*5F=7020 NOW SUBSTITUTE (42-F) FOR T & SOLVE FOR F
45*3(42-F)+45*5F=7020
135(42-F)+225F=7020
5670-135F+225F=7020
90F=7020-5670
90F=1350
F=1350/90
F=15 NUMBER OF 5 CREDIT COURSES.
T=42-15
T=27 NUMBER OF 3 CREDIT COURSES.
PROOF
45*3*27+45*5*15=7020
3645+3375=7020 
7020=7020