Fruit Seller

For every 10 apples he buys, it costs him 5 rupees. For every 10 apples he sells, he collects 6 rupees.

Thus, he makes 1 rupee per 10 apples, so he makes 0.10 per 1 apple. Since each apple costs him 0.50 rupees, his percentage profit is $\frac{0.1}{0.5} = 20\%.$

A fruit seller buys 2 apples for a rupee that is 1 apple for 0.5 rupees

So he buys 5 apples for 2.5 rupees
His percentage profit = (0.5/2.5)x100=0.20=20%

I'm not sure this is an accurate way to word a question. If he buys 2 and sells 5, he needs to have acquired an additional 3 apples somehow. For all we know, he may have bought them for 2 rupees or more, thus cancelling out his net profit.