Buying two more parsnips and three fewer turnips does not change the total cost and hence two parsnips cost the same as three turnips. Instead of six parsnips therefore, Baldrick could have bought nine turnips, and together with seven turnips, this makes a total of 16 turnips. Alternatively he could have bought twelve turnips instead of eight parsnips and , together with four turnips, this
makes sixteen turnips.
This problem is taken from the UKMT Mathematical Challenges.