One grocery clerk can stock a shelf in 40 min, whereas a second clerk requires 20 min to stock the same shelf. How long would it take to stock the shelf if the two clerks worked together?

Question

Is this how you do this:

t/40 + t/24 =1
t*3/40*3 + t*5/24*5 =1
3t/120 + 5t/120 =1
(8t/120)*120 = 1(120)
8t/8 = 120/8
=15

Answer 1

No,

rate of first = 1/40
rate of second = 1/20
combined rate = 1/40 + 1/20 = 3/40

so time for job = 1/(3/40) = 40/3 minutes
or 13 1/3 minutes