−15 ÷ 5 = A number line going from negative 30 to positive 30. An arrow goes from 5 to 10, from 10 to 15, from 15 to 20.

Emily used the number line to find the quotient for -15 ÷ 5. What errors did she make? Select three correct answers.
The arrows should point in the negative direction.
The small arrows should end in the same place as the large one.
The arrows should end on negative 15.
The arrows should start at zero.
There should be 5 small arrows.

Aprisoner is trapped in a cell containing three doors. the first door leads to a tunnel that returns him to his cell after two days of travel. the second leads to a tunnel that returns him to his cell after three days of travel. the third door leads immediately to freedom. (a) assuming that the prisoner will always select doors 1, 2 and 3 with probabili- ties 0.5,0.3,0.2 (respectively), what is the expected number of days until he reaches freedom? (b) assuming that the prisoner is always equally likely to choose among those doors that he has not used, what is the expected number of days until he reaches freedom? (in this version, if the prisoner initially tries door 1, for example, then when he returns to the cell, he will now select only from doors 2 and 3.) (c) for parts (a) and (b), find the variance of the number of days until the prisoner reaches freedom. hint for part (b): define ni to be the number of additional days the prisoner spends after initially choosing door i and returning to his cell.