Two fathers and two sons decided to go to a shop and bought some sweets upon reaching. Each of them bought 1 kg of sweet. All of them returned home after sometime and found out that they had 3kg of sweets with them.

They did not eat the sweets in way, nor did they threw or lost anything. Then, how can this be possible?

The people who went to shopping were son, father and grandfather. Thus, there were only three people even though they can be called as two fathers and two sons. This is why only 3kg of sweets.