Elaine can knit 4 scarves in 20 days...4/20 = 1/5 of a scarf per day
Mario can knit 2 more scarves then Elaine can in 40 days....so Elaine, if she knits 1/5 scarves per day, in 40 days, she can knit 1/5(40) = 40/5 = 8 scarves. And Mario can knit 2 more....so Mario can knit 10 scarves in 40 days.....10/40 = 1/4 scarf per day.
so if Elaine can knit 1/5 of a scarf per day, it would take her 5 days to knit 1 scarf....1/5 * 5 = 1
and if Mario can knit 1/4 of a scarf per day, it would take her 4 days to knit 1 scarf....1/4 * 4 = 1
so the difference in time is 1 day.....because it would take Elaine 5 days to knit 1 scarf, and it would take Mario 4 days to knit 1 scarf
