Probability 1

[Sdubord stats]

60% of students comes to school by bike.

A. If we pick 10 students in a group of 30 students and put them back in the lot, what is the probability that the first 7 will come by bike and the 3 other using an other transportation?

B. If we pick 10 students in a group of 30 students and DONT put them back in the lot, what is the probability that 7 will come by bike?

Thanks, i have difficulty to differentiate the 2.

 

[Cubist]

The question doesn't seem very clearly written. This is my interpretation...

In A each student is picked with replacement. So a student is picked, their method of transport is noted, and then the student re-enters the group of 30 (or the "lot" as they call it). This is repeated 10 times. This means there will still be 30 students in the lot after all 10 have been picked.

In B each student is picked without replacement, therefore after 10 have been picked there will only be 20 remaining in the lot. The same student can't be picked more than once.