Answer
The two events are $\approx141.429$ seconds apart on C.
Work Step by Step
For (b) we already have the ratio of B to A, so all we need is the ratio of C to B
$\frac{C}{B}=\frac{50s}{175s}=\frac{2}{7}\approx0.286...$
Then, we multiply this value by the answer we got for 13(a)
$495s\times\frac{2}{7}\approx141.429s$
and this gives us the quantity for C
