can someone double check this answer for me?

Question

I got 60/11

Ingrid can rake our lawn in 90 minutes and i can do it in 60 minutes. If ingrid rakes for 15 minutes before i join her, how long will it take us to finish?

Answer 1

Let X = the amount of lawn raked. You want it to equal 1.

Ingrid rakes at a rate of R1 = 1/90 lawn per mminute

The other person rakes at a rate of 1/60 lawn per minute

Let T be the time measured after the second person starts.

(1/90)(T + 15) + (1/60)T = X = 1

(5/180)T + 1/6 = 1
(5/180)T = 5/6 = 150/180
T = 50 minutes