If 4 people can do 1/2 the job in 2hr, 30 min [ 150 min], then it must take them 300 min to do the whole job.
And we can use the "see- saw" principle, here.
4 * 300 = 10 * (time to do the same job) .... divide both sides by 10
4*300/ 10 = 120 minutes to do the whole job for a crew of 10 people
Then, 1/3 of the job will take a crew of 10 people 40 minutes to complete
