Combining both statements you can find some value for the speed of R. This has to be less than 19 since we know how much time P+Q take and we know that P+Q+R is less than 19 i.e. 9hours.

Let's assume that Q did no work, then at the very least P takes 19hrs to finish the wall all by himself. If P's speed was less than R then time taken for P+Q would be a lot less than 19. So P is slower than R. Even Q is slower than R

Hence R is the fastest.