Probability

1. There are 20 students in Mrs. Church’s fourth grade. Mrs. Church had them all put their names on an index card. She’ll draw a name to see who will get to teach a math lesson on Friday. Carlos hopes he’ll be chosen. What chance does he have?

2. Mr. El just bought a box of Girl Scout cookies. He told his class, “I’d like to eat this box of thin mints all by myself. It’s one of my most treasured annual traditions! But I could share the cookies with all of my dear students. It’s only fair that I give you a chance to help me eat them. Here’s what we’ll do: There are 29 blue marbles in this cloth bag. There is one red marble. I’ll have David reach in, without peeking, and pull out a marble,. If it’s a red marble, I’ll share. If it’s blue, I’ll be forced to eat them all.

Under this plan, what chance does the class have to share the cookies?

3. Mr. El then admitted that he really was going to give the kids a better chance. There were actually 20 blue marbles and 20 red marbles in the bag. That would give the kids a better chance to win.

What was the chance that they would get to share the cookies under this plan?

4. David drew a blue marble. But Mr. El felt bad when he saw the sad faces on his 22 students’ faces. “OK, I’ll give you guys one more chance,” he said. “Each student can take out one marble. If anyone gets a red marble, I’ll share.”

What chance did the class have of winning this time?