Question

Wilma bought 4 boxes of Crunch-A-Lot cereal. One out of every 5 boxes has a coupon for a free box of Crunch-A-Lot. What is the probability that Wilma got at least 2 coupons?

Answer 1

4/5 chance of getting thre coupon

Answer 2

The answer is about 2.6% but i don't know how they got that answer..

Answer 3

let me do it again

Answer 4

coupon in 1 box = 1/5
no coupon in 1 box = 4/5
no coupon in 5 boxes = (no coupon in 1 box)^5
no coupon in 4 boxes = (no coupon in 1 box)^4

exactly 1 coupon in 5 boxes = no coupon in 4 boxes * coupon in 1 box

neither no coupon in 5 boxes nor exactly 1 coupon in 5 boxes = 1 - no coupon in 5 boxes - exactly 1 coupon in 5 boxes

Answer 5

= 1 - (4/5)^5 - (4/5)^4 * (1/5)

Answer 6

looks good to me
at least 2 means not none and not one

Answer 7

The answer is about 2.6% but i don't know how they got that answer.
coupon in 1 box = 1/5
no coupon in 1 box = 4/5
no coupon in 5 boxes = (no coupon in 1 box)^5
no coupon in 4 boxes = (no coupon in 1 box)^4

exactly 1 coupon in 5 boxes = no coupon in 4 boxes * coupon in 1 box

neither no coupon in 5 boxes nor exactly 1 coupon in 5 boxes = 1 - no coupon in 5 boxes - exactly 1 coupon in 5 boxes
= 1 - (4/5)^5 - (4/5)^4 * (1/5)
.

Answer 8

wait, I solved it for if she bought 5 boxes, and forgot to account for the fact that there's more than one way for her to win only one
1 - (4/5)^4 - 4*(4/5)^3 * (1/5)

=18.08%

2.6% is the right answer to "What's the probability she got 3 coupons?"