Fraction problem

Question

There are 25 students in Roses’s Speech class. Rose has not yet been called on to give her required oral report, but 10 other students have given their reports. If 2 students are absent, what is the probability the Rose will be called on to give her report assuming one student gives a report each day.

Answer 1

25 students, 10 gave their report, 2 are absent, that leaves 13 for the teacher to choose from.

25-10-2 = 13

1/13 chance she'll be called on = about 7.69% chance

Unless the 2 students who are absent are among the 10 that already gave their report. that would leave 15 students to choose from and probability 1/15 = about 6.67% chance of getting chosen

Or, if just 1 of the 2 students who are absent was among the 10, then there's 14 who the teacher could select, with probability 1/14 = about about 7.14% Overall, that's close to the actual probability of being selected.

average of 1/13, 1/14 and 1/15 is add them up, divide by 3

put them over a common denominator 13x14x15 = 2730

14x15/2730 + 13x15/2730 + 13x14/2730 = 210/2730 + 195/2730 + 182/2730 = 587/2730 divide by 3 to get

587/8190 = about 7.17% chance of being selected to present the paper