Question

8 workers together at the same rate finished a painting job in 5 hours. hour long would it take 12 such workers working at the same rate to finish the same job? each worker finished____(fraction) of the job in one hour. 12 workers finished_____(fraction) of the job in one hour, they finished the job in_____hours

Simple.
it takes 40 'workerhours' to do the job.

so if 12 'workers' do it, it would take 40/12 hours or 3 hours and 20 minutes.