If Nathan and Rebecca work together on a project, they will complete it in 10 days. If Rebecca were to work with Ella on the same project, they will take a day longer. If Ella and Risa work together, they will take 90% more time than Nathan and Rebecca. If Nathan and Risa were to work together, how many more days will they take to complete the project compared to Rebecca and Ella working together?
Time taken by Nathan and Rebecca in percent = 100%
Time taken by Ella and Risa = 100% + 90% = 190%

Time taken by Nathan and Rebecca = 10 days ---- (1)
Time taken by Rebecca and Ella = 11 days ---- (2)
Time taken by Ella and Risa = 190% x 10 = 19 days ---- (3)

(1) + (2) + (3)
Nathan + Rebecca + Rebecca + Ella + Ella + Risa → 10 + 11 + 19 = 40 days
Nathan + 2 (Rebecca + Ella) + Risa → 40 days
Nathan + Risa + 2 x 11 → 40 days
Nathan + Risa → 40 - 22 = 18 days

More days to complete the project for Nathan and Risa compared to Rebecca and Ella
= 18 - 11
= 7

Answer(s): 7