Probability, i forgot how to do this type of problem

[Guest]

:?: Please help me-

A bookshelf contains seven math textbooks and three science textbooks. If two textbooks are drawn without replacement, what is the probability both books are science texbooks.

 

[xerip]

So we want the denominator to be all the possibilities or 2 textbooks drawn at random. This is 10*9= 90.

1- Choose the first textbook (10 possibilities)
2- Choose the second textbook (9 possibilities)

Multiply the possiblilities, so 10*9 = 90 possibilities



Then the numerator is the possible combinations of having 2 science textbooks.

Again, we do the same:

1- Choose the first science textbook (3 possibilities)
2- Choose the second science textbook (2 possibilities)

3*2 = 6


so the probability is 6/90 = 1/15 ~ .0667 = 6.67%

I think this is right. Is this any good?

 

[tkhunny]

More simply:

\(\displaystyle \L \frac{3}{10}*\frac{2}{9}\.=\.\frac{6}{90}\)