Workout Plan A lasts A hours while workout Plan B lasts B hours.
On Monday: 3A+5B=9; on Tuesday: 12A+2B=9.
Multiply Monday’s by 4: 12A+20B=36, so 12A=36-20B.
So we can substitute for 12A on Tuesday:
36-20B+2B=9, 36-18B=9.
Divide through by 9:
4-2B=1, 2B=3, so B=3/2=1.5 hours.
So, since 12A+2B=9, 12A+3=9, 12A=6, A=6/12=0.5 hours.
Plan A is 0.5 hours (30 mins) and Plan B=1.5 hours (90 mins).