The addition rule, and the general multiplication rule

Grade 11 Math Worksheet

Circle the best answer.

1. 1.

   P(A) = 0.5, P(B) = 0.6 and P(A or B) = 0.8. Bo tests whether A and B are independent. Which conclusion fits?

   • Not independent: P(A and B) = 0.8 − 0.5 = 0.3 differs from P(B) = 0.6
   • Independent: P(A and B) = 0.5 + 0.6 − 0.8 = 0.3, and 0.5 × 0.6 = 0.3
   • Not independent: A and B overlap by 0.3, so they share outcomes
   • Not independent: P(A or B) = 0.8 differs from 0.5 × 0.6 = 0.3
2. 2.

   P(A) = 0.5, P(B) = 0.6 and P(A and B) = 0.18. Esme finds P(A given B). What does it show?

   • 0.18 × 0.6 = 0.108, so B makes A very unlikely
   • 0.18 ÷ 0.6 = 0.3, below P(A) = 0.5, so B lowers the chance of A
   • 0.18 ÷ 0.5 = 0.36, below P(B) = 0.6, so A lowers the chance of B
   • 0.18 ÷ 0.6 = 0.3, close to P(A) = 0.5, so they are independent
3. 3.

   P(A) = 0.4, P(B | A) = 0.6 and P(B) = 0.5. Jun finds P(A given B). Which is right?

   • 0.4 ÷ 0.5 = 0.8, dividing the two single chances
   • 0.6, the same as P(B | A)
   • 0.4 × 0.6 = 0.24, the chance of A and B
   • 0.4 × 0.6 ÷ 0.5 = 0.48
4. 4.

   A jar has 5 red and 4 green marbles. Kira draws two without putting any back. What is P(two different colors)?

   • 5/9 × 4/8 + 4/9 × 3/8 = 4/9, both the same color
   • 5/9 × 4/8 = 5/18, red then green
   • 5/9 × 4/8 + 4/9 × 5/8 = 5/9
   • 5/9 × 4/9 + 4/9 × 5/9 = 40/81
5. 5.

   The same jar holds 5 red and 4 green marbles. Luz draws two without putting any back. What is P(at least one red)?

   • 1 − 4/9 × 4/9 = 65/81
   • 5/9 × 4/8 = 5/18, two reds
   • 1 − 4/9 × 3/8 = 5/6
   • 4/9 × 3/8 = 1/6, no red marbles
6. 6.

   A bag has 3 red and 5 green marbles. Milo draws two without replacement. What is P(the second marble is red)?

   • 2/7, because the first marble was red
   • 3/7, because the first marble was green
   • 3/8 × 2/7 + 5/8 × 3/7 = 3/8
   • 3/8 × 2/7 = 3/28, red then red
7. 7.

   A drawer holds 6 black and 4 white socks. Noah pulls three in the dark, keeping each. What is P(all three black)?

   • 6/10 × 5/10 × 4/10 = 3/25
   • 6/10 × 5/9 × 4/8 = 1/6
   • (6/10)³ = 27/125
   • 3 × (6/10 × 5/9 × 4/8) = 1/2, counting three orders
8. 8.

   P(A) = 0.5 and P(B) = 0.3. Pia asks how small and how large P(A or B) can be. Which range is right?

   • From 0.5 up to 1: the two events could cover everything
   • From 0.5 up to 0.8: B inside A gives 0.5, no overlap gives 0.8
   • From 0.3 up to 0.8: the smaller chance is the least it can be
   • Exactly 0.8, since A and B are different events
9. 9.

   P(A) = 0.4, P(B) = 0.35 and P(A and B) = 0.15. Ren finds P(neither A nor B). Which is right?

   • 1 − (0.4 + 0.35 − 0.15) = 0.4
   • 0.4 + 0.35 − 0.15 = 0.6, the chance of A or B
   • 1 − (0.4 + 0.35) = 0.25
   • 1 − 0.4 − 0.15 = 0.45
10. 10.

    Machine A makes 60% of parts, 2% defective. Machine B makes 40%, 5% defective. Zee picks a part at random. What is P(defective)?

    • 0.6 × 0.02 = 0.012, from machine A
    • 0.02 + 0.05 = 0.07
    • 0.6 × 0.02 + 0.4 × 0.05 = 0.032
    • 0.6 × 0.05 + 0.4 × 0.02 = 0.038