How many minutes will it take 5 people to stack 250 chairs if each person can stack 20 chairs a minute?

Ok, so here we need to set up a proportion.

20*5 = 100

So in a minute, the 5 people have 100 chairs stacked.

1/100 = x/250

Cross multiply

1*250 = 100x
250 = 100x
250/100 = x
x = 2.5

So it would take 5 people 2.5 minutes to stack 250 chairs. 

Final 2.5 minutes