Baldrick can afford to buy either 6 parsnips and 7 turnips or else 8 parsnips and 4 turnips. Both options leave him with no change whatsoever. If, however he bought only his beloved turnips, how many could he afford?
If you liked this problem, here is an NRICH task that challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.