Each piglet in a litter is fed exactly one-half pound of a mixture of

@subhashghosh: As requested by you, I will try and explain the logic used in this question.

For now, forget the data given in the question. Consider this:

10% of Oats and 20% of Barley was fed to piglet A.

My question is - Of the total food (Oats + Barley) what percentage was fed to piglet A?
It will depend on the ratio of Oats and Barley. If the mixture had only oats, the piglet was fed 10% of total food. If the mixture had only Barley, the piglet was fed 20% of the mixture. If the mixture had half oats and half barley, the piglet was fed 15% of the mixture. If the mixture had 1 part Oats for every 4 parts Barley, the piglet was fed 18% of the mixture (it is just weighted average). Whatever the case, the piglet was fed more than 10% of total food and less than 20% total food if the mixture had both Oats and Barley.

If this sounds good, consider data given in the question – the piglet was fed 25% Oats and 16.66% Barley (1/4 Oats and 1/6 Barley). So definitely, the piglet was fed more than 1/6 of the total mixture and less than ¼ of the total mixture (as reasoned above). The total food mixture was split equally among all the piglets. Number of piglets has to be an integer, say n. Each piglet gets the same amount of food i.e. 1/n of the total food. But each piglet also gets less than ¼ of total food and more than 1/6 of total food. The only integral value for n such that 1/6 < 1/n < ¼ is 5.