Question

Jane works twice as fast as her daughter Anna. If it takes 15 minutes to clean the kitchen together, how long would it take Anna to clean the kitchen by herself

Answer 1

suppose Jane can finish cleaning by herself in x minutes, every minute she does 1/x portion of the work
Anna can finish cleaning by herself in 2x minutes, every minute she finishes 1/(2x) portion of the work
together, they finish (1/x +1/2x) portion of the work
in 15 minutes, they finish the whole work: 15/x +15/2x =1
solve for x: 30/2x +15/2x =1. x=22.5
2x=45
Anna can finish cleaning by herself in 45 minutes.