pedro and juan can do a piece of work in 3 3/7 days. if pedro works in 3 days and juan in 4 days, they will finish the work. find the time required by each to do the work.
still can't figure out what you mean by
pedro works in 3 days and juan in 4 days
does that mean that Pedro works for 3 days, and then Juan works for 4 days?
It appears that you are saying it takes Pedro 3 days, and it takes Juan 4 days. Then you ask how long it takes each to finish.
In the first sentence you appear to be saying that
1/j + 1/p = 1/(3 3/7)
Try explaining this a bit more, ok?