how to calculate a percentage of a budget

Question

my department at work has a monthly budget for drinks and snacks. a few days each month, another department uses our space and our supplies. how do i calculate what to charge them for the drinks and snacks they use, factoring in that they only use that space for a limited amount of days and their department is a smaller amount of people (both those figures are known quantities) many thanks

Answer 1

You want to count person-days.

Suppose your department has $18$ people and uses the space $25$ days while the guest department has has $10$ people and uses the space $5$ days. that's $$ 18 \times 25 + 10 \times 5 = 500 \text{ person-days} $$ worth of refreshments, assuming everyone consumes about the same amount.

Their share is $$ \frac{50}{500} = 0.10 = 10\% $$ of the cost.

(I imagined numbers to make the arithmetic easy.)