Let a and b be separated by s meters, and c needs t to catch up with a, so c needs t+ 10 to catch up with b,

Then the walking distance of C catching up with A is equal to the distance between A and B plus the walking distance of A: 100t=s+60t.

The distance for C to catch up with B is equal to the distance between A and B plus the distance of B:100 (t+10) = s+80 (t+10).

Solution: t= 10min, s=400m, that is, the distance between a and b is 400m.

Thank you, I wonder if you can understand!

thank you