last week you spoke with 800 customers in 40 hours that is an average of _________ customers every 30 minutes

The long way to do this is to convert hours into minutes, or minutes into hours. So multiply 40 by 60, and you get 2,400 minutes. So in 2,400 minutes, you spoke to 800 customers. Divide 800 by 2400, and you get .33333 repeating, which is how many customers you get a minute. Multiply this by 30, and you get 10, which means that every 30 minutes, you speak to 10 customers.
You can do this converting minutes to hours by changing 30 into .5, because 30 minutes is .5 hours. Divide 800 by 40 and you get 20, multiply 20 by .5 to get 10.