cards numbered 1-10 are colored red and cards 11-20 are colored black. What is the probibility of drawing a card with a 3 or a black card

Answer 1

P(3) = 1/20

P(B) = = 1/2

P(3orB) = P(3)+P(B)-P(3&B)

= P(3) +P(B) - P(3)P(B)

= 1/20 + 1/2 - (1/20)(1/2)

= 1/20 + 1/2 - 1/40

= 2/40 + 1/2 -1/40

= 1/2 +1/40

= 20/40+1/40

= 21/40

= .55 = 55%

It might help to draw some Venn diagrams