The time t that it takes for a group of volunteers to construct a house varies inversely as the number of volunteers v. if 20 volunteers can build a house in 62.5 working hours, how many working hours would it take 15 volunteers to build a house? round to the nearest tenth.

If 20 volunteers can build a house in 62.5 working hours, then
the job actually takes (20 x 62.5) = 1,250 volunteer-hours of work.
(That's how long it would take 1 volunteer to do the job completely
on her own.)

If there are only 15 volunteers, then they would need
(1250/15) = 83-1/3 hours to build the house, working
together.

What the heck don’t understand