let

x = the number of hours that 1 worker needs to finish the project alone

1/x = the rate of 1 worker per hour

4 workers can finish a project in 9 hours:

4(1/x) = 1/9

by solving we find:

x = 36 hr

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3 workers will take 3 times shorter time than 1 worker or:

(1/3)*(36) = 12 hr

it will take 12 hours for 3 workers to finish a project.