The horse fair problem

This is essentially two separate deals, on each of which he makes £10, but people often think he has ‘lost’ £10 when he buys the horse back for £80. If you pose the same question as ‘A farmer buys a black horse for £60 and sells it for £70. A year later he buys a white horse for £80 and sells it for £90, how much does he make?’ it seems much easier, even though it’s the same question!