Two events and complements
Name: _____________________ Date: __________
Show your thinking with numbers, words, or a drawing.
1. A bag has 8 counters. The probability that you take a red counter is 5/8. What is the probability that you do NOT take a red counter?
2. A bag has 9 counters. The probability that you take a red counter is 4/9. What is the probability that you do NOT take a red counter?
3. A bag has 8 counters. The probability that you take a red counter is 6/8. What is the probability that you do NOT take a red counter?
4. The probability that it rains tomorrow is 8/10. What is the probability that it does NOT rain tomorrow?
5. The probability that it rains tomorrow is 3/10. What is the probability that it does NOT rain tomorrow?
Answer key · Two events and complements
1. 3/8
An event and its complement (the event not happening) always add up to 1.
1 = 8/8, so take away 5/8: 8/8 − 5/8 = 3/8.
P(not) = 3/8.
2. 5/9
An event and its complement (the event not happening) always add up to 1.
1 = 9/9, so take away 4/9: 9/9 − 4/9 = 5/9.
P(not) = 5/9.
3. 1/4
An event and its complement (the event not happening) always add up to 1.
1 = 8/8, so take away 6/8: 8/8 − 6/8 = 2/8.
P(not) = 2/8 = 1/4.
4. 1/5
An event and its complement (the event not happening) always add up to 1.
1 = 10/10, so take away 8/10: 10/10 − 8/10 = 2/10.
P(not) = 2/10 = 1/5.
5. 7/10
An event and its complement (the event not happening) always add up to 1.
1 = 10/10, so take away 3/10: 10/10 − 3/10 = 7/10.
P(not) = 7/10.
Try it together
Choose a question from the printable. Use objects or a drawing to show your thinking, then explain your method to someone at home.