Martin randomly selects 3 days of the week to work at his uncle's farm. what is the probability he selects monday, tuesday, and friday in any order?

A week has 7 days. That is, number of total days, n = 7
Number of Mondays in a week = 1
Number of Tuesdays in a week = 1
Number of Fridays in a week = 1

This is a case of selecting without replacement in that order.

P(Monday) = 1/7
P(Tuesday) = 1/(n-1) =1/(7-1) = 1/6
P(Friday) = 1/(n-2) = 1/(7-2) = 1/5

P(Monday, Tuesday, and Friday) = (1/7)*(1/6)*(1/5) = 1/210