Answer:
they can finish
[tex] \frac{1}{15} + \frac{1}{12} \\ [/tex]
in one hour
[tex] \frac{4 + 5}{60} \\ \frac{9}{60} [/tex]
in 6 hours they can finish
[tex] \frac{9}{60} \times 6 \\ \frac{54}{60} [/tex]
so the left work
[tex] \frac{60 - 54}{60} [/tex]
[tex] \frac{6}{60} [/tex]
they can perform
[tex] \frac{3}{60} work in60 \: minutes \div 3[/tex]
so the can perform
[tex] \frac{6}{60} work \: in \: 20minutes \times 2[/tex]
the take 6 hours and 40 minutes
