15. Carrie the cat and Barrie the bat together weigh $4000 \mathrm{~g}$ more than Rollie the rat. Barrie and Rollie together weigh $2000 \mathrm{~g}$ less than Carrie. Carrie and Rollie together weigh $3000 \mathrm{~g}$ more than Barrie. What is the weight, in grams, of Rollie the rat?
A 250
B 500
C 750
D 1000
E 1250
15. B Let the weights, in grams, of Carrie, Barrie and Rollie be $c, b, r$ respectively.
Then: $c+b=4000+r ; \quad b+r=c-2000 ; \quad c+r=3000+b$.
Adding these three equations gives: $2 c+2 b+2 r=5000+r+c+b$. So $c+b+r=5000$.
Hence $c+b=5000-r$.
Substituting in the first of the above equations: $5000-r=4000+r$. Therefore $2 r=1000$. Hence the weight of Rollie the rat is $500 \mathrm{~g}$.