Answer is 25 days to build the whole road
Step by step
Let’s call total number of days N
3/5 of N = 15 days
3/5 x N = 15
Divide both sides by 3/5
(When you divide fractions flip the divisor and multiply)
3/5 x 3/5 N = 15 x 5/3
Simplify
N = 25 days
If you took 2/5 of a road x 25
2/5 x 25 = 50/5 = 10
2/5 road = 10 days
3/5 road = 15 days
Total days = 25
Problem solved