If 6 workers complete a certain task in 12 days, how many workers are needed to finish the same task in 18 days?

The task requires $6 \text{ workers" xx 12 " days" = 72 " worker-days}$
Spread over $18 \text{ days}$, this task would require
color(white)("XXX")(cancel(72)^4 " worker"cancel("-days"))/(cancel(18)cancel("days")) = 4 " workers"