Extra workers needed to finish tilling plots

A contractor must till $11$ equal plots. Four workers spend $5$ hours finishing the first $2$ plots. The remaining $9$ plots must be done within the next $12$ hours. Assuming every worker tills at the same constant rate no matter how many are on site, what is the fewest extra workers the contractor must add?

4. Two plots took $4\times5=20$ worker-hours, so one worker tills $\tfrac1{10}$ plot per hour. With $4+n$ workers for $12$ hours we need $\tfrac{12(4+n)}{10}\ge9$, i.e. $n\ge3.5$, so $n=4$ (eight workers need $11.25$ hours).