Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
If they did not specify the holding fuel consumption rate, then a 50 litre value appearing within the Trip Fuel calculation would make sense since the holding fuel would have to be calculated at the provided cruise consumption rate. This is the only source I can find for a value of 50 to originate from.
However, they did specify a holding consumption rate of 30 l/hr (as you rightly pointed out) within the question, so I agree that the 60 min holding fuel required at A should be 30 litres. I suspect it is an error in the working that did not account for the fact that the holding fuel consumption rate was given.
No worries. If it helps at all, I just did the problem now and got 187.6nm, which is very close to your answer (the minor difference is likely due to any rounding errors or human error in obtaining the headwind component from the flight computer. I got a headwind component of 28 kts).
I wouldn't stress too much over it (at least till Stuart/Bob confirms)