In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
Bob = 7 dog leashes in 2 hours
Nina = 4 dog leashes in 1 hour
60 leashes together
t = ?
Step 02:
Bob = 7 dog leashes / 2 hours
Nina = 4 dog leashes / 1 hour
Time = 60 leashes * ( 2 hours / 7 leashes + 1 hour / 4 leashes)

time = 32.14 hours
The answer is:
The time together is 32.14 hours