# last question

• Jan 23rd 2007, 04:07 PM
mauro21pl
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
• Jan 23rd 2007, 04:15 PM
ThePerfectHacker
Quote:

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.
${6\choose 2}+{7\choose 2}+{5\choose 2}$
Quote:

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
${6\choose 1}\cdot {7\choose 1}+{6\choose 1}\cdot {4\choose 1}+{7\choose 1}\cdot {4\choose 1}$