# Math Help - last question

1. ## last question

A student has to sell 2 books from a collection of 6 math, 7 science, and 4 economics books. How many choices are possible if
a)both books are to be on the same subject
b)the book are to be on different subject
thanks

2. Originally Posted by mauro21pl
A student has to sell 2 books from a collection of 6 math, 7 science, and 4 economics books. How many choices are possible if
a)both books are to be on the same subject
The possibilities are:
2 from 6 math
2 from 7 science
2 from 5 economics
You shall find the number of combinations for each one add them them together for they are disjoint.
${6\choose 2}+{7\choose 2}+{5\choose 2}$
b)the book are to be on different subject
thanks
The possibilities are:
1 from 6 maths, 1 from 7 science
1 from 6 maths, 1 from 4 economics
1 from 7 science, 1 from 4 economics
${6\choose 1}\cdot {7\choose 1}+{6\choose 1}\cdot {4\choose 1}+{7\choose 1}\cdot {4\choose 1}$