Rate of work problem

I forgot how to solve this problem:
N and L are able to set up 300 chairs in 10 minutes. N works twice as fast as L. How long would it take N to set up the chairs alone?

Make a fraction to represent the workdone by each person.

time working together/time needed if alone

If N works twice as fast, L will take twice as long.
N = 10/x

L = 10/2x

Add the fractions. If the job gets done, the answer is 1.

10/x + 10/2x = 1

10/x + 5/x = 1

x = 15

It would take N 15 minutes to do it alone.

N can set up 200 chairs in 10 minutes, and L can set ut 100 chairs in 10 minutes. So N will set up 300 chairs in 15 minutes.

2L*10+L*10 = 300
L = 10
N = 20 chairs/min
20t = 300
t = 15 mins