How do you calculate what percentage of a number another number is?

I am confronted with this problem regularly and either I am away or someone else gets it done in my stead, never with me finding out how to do it. I am afraid to look stupid at work being one of the bosses, but I need to know this, and high school was long
Update: Like I came into breakfast sales of $170 and since the changes I have implemented the sales are now $1300... How do I determine the increase in percentage?