Probability

By all of them? By at least one of them?

What is the rule that a student is chosen among the three?

The probability that they all answer correctly is .192.
The probability that at least one of them answers correctly is .952.

100%.

You add each percentage together and divide by 300 (the total percentage)
So it would be 80 + 60 + 40 = 180 so it is 180/300 which leaves 60, so there is a 60% chance. Hope this helps!!

I believe MarkR is correct.

80% chance leaves 20% chance of failure.
60% chance of getting right after that leaves 8% of failure.
40% chance of getting it right after that leaves 4.8% chance of failure.

Chance of getting the right answer: 95.2% (.952 probability)

Another way to do it ...
.8+.6+.4 = probability that anyone gets it right, but this incorrectly includes the cases where 2 people get it right. so you need to subtract these cases, which is .8*.6 + .8*.4 + .6*.4 ........... but wait a minute, that takes out too much .. you have to put back the case where all 3 get it right.

.8+.6+.4 - (.8*.6 + .8*.4 + .6*.4) + .8*.6*.4 =

1.8 - 1.04 + .192 = .952 as previosly noted.

This solution can be seen more easily with a Venn diagram.

95.2%