Emma can grade 10 papers in 20 minutes = 1 paper in 2 minutes = 5 papers in 10 minutes.

Grace can grade 15 papers in 25 minutes = 3 papers in 5 minutes = 6 papers in 10 minutes.

Together they can grade 11 papers every 10 minutes. That's less than 1 minute per test so you know your final answer will be less than 30 minutes.

30 tests / (11 tests / 10 minutes) = 30 tests * (10 minutes / 11 tests) = 300 / 11 minutes = 27.2727... minutes. Another helpful hint when doing these type of questions is to always include your units, which is "tests" and "minutes" in this case. If your final answer doesn't work out to have "minutes" as the unit, you know you messed up somewhere.

]]>i did:

E = 10p in 20m

G = 15p in 25m

30 p in how many m

10p + 15p = 25p

20m + 25m = 45m

emma can grade 5p in 10m

so

10p + 15p + 5p = 30p

20m + 25m + 10m = 55m

so answer is 55m