another way of showing would be
since it says the same number of counters for both ratios, as above did, add up each ratio. For the start, the ratios add up to 18. For the 2nd, they add up to 24. The LCM (any multiple works) of 18 and 24 is 72. You can assume there are 72 counters (this is show that, so plugging in figures should be allowed).
5:6:7 of 72 is 20:24:28. Rob has 20 counters at the start.
7:8:9 of 72 is 21:24:27. Rob now has 21. 21>20, so he has more than he started with.