Answer:

30 minutes

Step-by-step explanation:

If Kelly use x minutes and Anna use y minutes to clean the room alone,

Then,x + 15=y

(1/x+1/y)×18=1

As 1 and 2

x=-9 and y=6

Or x=30 and y=45

Because x > 0

So it take Kelly 30 minutes to clean the room alone.