A boy and three girls do as many sums in 5 minutes as 3 boys and 5 girls do in 4 minutes. Who works faster?

a. neither

b. girls

c. both equal

d. boys

First this problem must be assuming that all boys do "sums" at the same rate and that all girls do "sums" at the same rate. Let "b" be the rate, in 'sums per minute", at which boys work, and let "g" be the rate at which girls do the same sums. "A boy and three girls" can do b+ 3g sums per minute so 5(b+ 3g)= 5b+ 15g sums in 5 minutes. "3 boys and 5 girls" can do 3b+ 5g sum per minute so can do 4(3b+ 5g)= 12b+ 20g sums in 4 minutes. We are told thoe are the same: 5b+ 15g= 12b+ 20g. Subtract 5b+ 15g from both sides: 7b+ 5g= 0. That's non-sense. It says that one of b or g must be negative! The information given cannot be correct and we cannot conclude anything. If I really had to give an answer, it would be "a. neither" which is apparently different from "c. both equal".