A bag contains a total of 20 books on physics and mathematics, Any possible combination of books is equally likely. Ten books are chosen from the bag and it is found that it contains 6 books of mathematics. Find out the probability that the remaining books in the bag contains 3 books on mathematics.

Explanation

` r= 3 `
` ar^( n-1) = 486`
and ` S_(n) = (a(r^(n) -1))/(r-1) = 728 `
` rArr (ar^(n) -a)/(r-1) = 728`
` rArr ((486 xx3) -a)/(3-1) = 728`
` rArr 1458 - a = 1456`
` rArr a = 1458 - 1456 `
` therefore a = 2 ` .