Question 711313
1/48+1/x=1/30
1/x=1/30-1/48
1/x=(48-30)/30*48
1/x=18/1440
18x=1440
x=1440/18
x=80 minutes for the brother working alone.
Proof:
1/48+1/80=1/30
(80+48)/48*80=1/30
128/3840=1/30
1/30=1/30