Question

39 people can repair a road in 12 days, working 5 hours a day. In how many days will 30 people working 6 hours a day complete the work?

Answer 1

Let the number of days required for 30 people working 6 hours a day be x.

Number of people	Time taken (in hours)
39	12$\times$5 = 60
30	x$\times$6 = 6x

Time taken and number of people are in inverse variation.
So, using arrow method:
$\frac{39}{30}=\frac{6x}{60}$
On cross multiplication,
$x=\frac{39}{30}\times \frac{60}{6}=13$
So, the required number of days is 13.