A man gives 25% of his land to his daughter and 30% of what is then left to his son, and keeps the rest for himself. What per cent of his land does he keep?

Take the land as 100. The daughter takes 25, leaving 75. The son takes 30% of those 75 = 22.5, so the man keeps 75 - 22.5 = 52.5%. Equivalently $0.75\times 0.70=0.525$. 45% is 100 - 25 - 30, which wrongly treats the son's 30% as a share of the whole holding instead of a share of the remaining three-quarters. 22.5% is the son's share alone, and 47.5% is what was given away.