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 answer: 2.5 minutes