You are at a party with a friend and 10 people are present including you and the friend. Your friend makes you a wager that for every person you find that has the same birthday as you, you get $1; for every person he finds that does not have the same birthday as you, he gets $2. Would you accept the wager?

There is a 1/365 chance that you share the same birthday with a random person, and 364/365 chance you don't. Don't take the wager.

What is the angle of a clock's hands when the clock shows 3:15?

360 degrees in a full circle. 90 degrees per quarter (that breaks up every 3 hours segment).

So that's 30 degrees per hour. Each hour can be broken up into 4 parts (one for every 15 minutes), 30/4 = 7.5 degrees.

The minute hand would point to 3, and the hour hand would be 1/4 of the way to 4. It is a quarter of the way to 4, because only 15 minutes (which is 1/4 of 60 mintes) have passed.

Say an advertiser makes $0.10 every time someone clicks on their ad. Only 20% of people who visit the site click on their ad. How many people need to visit the site for the advertiser to make $20?

Number of people needed to click to make $20:
10 people to make a $1
X 20
200 people to make $20

Those 200 are the 20% that clicked.

(20/100) * x = 200

20 * x = 20000
x = 1000

So 20% of a 1000 people is 200. So a 1000 people.