Question

John can paint the room in 4 hours and Jack can paint the room in 5 hours. If John starts to work alone at 1p.m. until 2 p.m. and then both John and Jack work together to paint the room, when will they finish the room? (Assuming they keep their usual speeds).

Answer 1

the anwer is $\frac{5}{3}$
they wil finish at time 3:40 pm
EXPLANATION:
john does $\frac{1}{4}$ in an hour
Jack does $\frac{1}{5}$ in an hour
therefore
$\frac{1}{4}+( \frac{1}{4} + \frac{1}{5} )t=1$
solve it
u get $t= \frac{5}{3}$
which is 100 minutes