# [SOLVED]IB5 probability from chart

#### karush

##### Well-known member
View attachment 1210

the numbers in () and boxes are mine, easy problem but still can make mistakes..

#### Klaas van Aarsen

##### MHB Seeker
Staff member
View attachment 1210

the numbers in () and boxes are mine, easy problem but still can make mistakes..
Hey karush!

Your table looks just fine and so is your answer to the first question.

However, for the second question, you have to consider what the total number of possibilities is. Apparently you thought it was a 100, but then you did not take into account that it is given that it is a boy. You should look only at the boys.

#### karush

##### Well-known member
Hey karush!

Your table looks just fine and so is your answer to the first question.

However, for the second question, you have to consider what the total number of possibilities is. Apparently you thought it was a 100, but then you did not take into account that it is given that it is a boy. You should look only at the boys.
see what you mean, so it should be $\displaystyle\frac{13}{38}$

#### Klaas van Aarsen

##### MHB Seeker
Staff member
see what you mean, so it should be $\displaystyle\frac{13}{38}$
Nope. Not yet.
It should be the number of boys that watch television divided by the total number of boys.
You just calculated the probability that a student is a boy, given that (s)he prefers to watch television.

#### karush

##### Well-known member
$\displaystyle\frac{13}{46}$