1) Let’s assume that the total work is 60 units (the LCM of 15 and 20).

A can do the work in 15 days, so his daily rate of work is 4 units/day (60/15).

B can do the work in 20 days, so his daily rate of work is 3 units/day (60/20).

When they work together, their combined daily rate of work is 7 units/day (4+3).

After 4 days of working together, they have completed 28 units of work (7x4).

The fraction of work that is left is (60-28)/60 = 32/60 = 8/15.

Therefore, the fraction of work that is left is 8/15.

2) Let’s assume that the total work is 48 units (the LCM of 16 and 12).

A can lay the track in 16 days, so his daily rate of work is 3 units/day (48/16).

B can lay the track in 12 days, so his daily rate of work is 4 units/day (48/12).

When they work together with C, their combined daily rate of work is x units/day.

After 4 days of working together, they have completed 4x units of work.

We know that A, B, and C together can complete the job in 4 days, so:

4(3+4+x) = 48

Simplifying this equation, we get:

28 + 4x = 48

4x = 20

x = 5

Therefore, the combined daily rate of work when A, B, and C work together is 5 units/day.

If C can do 5 units of work per day, then he can complete the job alone in 48/5 = 9.6 days (or approximately 9 days and 14 hours).

Therefore, C alone can lay the track in 9.6 days.