If the time it takes to build the shed is proportional to the amount of people building it then:
The proportionality constant is 9/4 (this value determines how much the time changes each time one person is added to the construction)
One third of 9 is 3, multiply the time by the proportionality constant to get the number of people needed:
3*(9/4)= 6.75
You'll need arround 6 people to construct the shed in one third of the time.
