a speed boat Factory can produce 12 boats in 8 days how many days will it take to factory to produce 30 speed boats

Assuming the factory is working at the same production rate, the number of days is proportional to the number of boats.

... 8 days / (12 boats) = x days / (30 boats)

Multiply by 30 boats, and you get

... x days = 8 days*30/12 = 20 days

It will take the factory 20 days to produce 30 boats.