Discrete math (macm 101 question)

i need to understand this notation:

permutation general formula : npr = n! / (n - r)! n stands for the total number of things we have, but what about r ?

Mary spent a total of $323.14 for a party. she spent a total of $200.22 on food, plus an additional $30.73 for each hour of the party. how long was the party?

Regular price is $77.00 and the discount is 5% find the sale price

Arefrigeration system at your company uses temperature sensors fixed to read celsius (°c) values, but the system operators in your control room understand only the fahrenheit scale. you have been asked to make a fahrenheit (°f) label for the high temperature alarm, which is set to ring whenever the system temperature rises above –10°c. what fahrenheit value should you write on the label