Question

Can someone help with this grade 9 math question?

This is the question:
It takes Laura 40 minutes to clean her room. When her mother cleans Laura's room it only takes 15 minutes. Which of these is closest to the time it would take to clean the room if they both worked together?
A. 11 min
B. 12 min
C. 13 min
D. 14 min

Your help is appreciated!

Answer 1

A.

Explanation:

I approached it as such:

I determined the rates at which they worked:
Laura: 1 room in 40 mins, so `1/40` rooms/ min
Laura's mom: 1 room in 15 mins, so `1/15` rooms/ min

Added their working rates together:
`1/40+1/15= 11/120 "rooms/ min"`

So to find their total mins per room, just reciprocate:
`120/11 "mins/room"approx 11 "mins"`