A certain list consists of 21 different numbers. If N is in the list and N is 4 times the average (arithmetic mean) of the other 20 numbers in the list, then N is what fraction of the sum of the 21 numbers in the list?
A: 1/6
21
There printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T working together at their respective constant rates, can do the same job in 5 hors. How many hours would it take R, working alone at its constant rate, to do the same job?
A: 20
對不起問題很多
