7 bags of rice, weighing 12kg, 15kg, 17kg, 20kg, 22kg, 24kg and 26kg respectively. A buys a bag first, and the rest is bought by B, C and D. It is known that the weight bought by B and C is exactly the same, twice that of D. Ask the weight of the bag of rice bought by A first.

Analysis because B and C bought the same weight, and both were twice as heavy as D.

So the weight bought by three people is five times that of Ding;

The total weight of seven bags of rice is12+15+17+20+22+24+26 =136 kg.

136 kg minus a multiple of 5, the rest is the weight bought by A..

Conversely, if you subtract the weight of the bag of rice bought by A from 136 kg, the remaining weight must be a multiple of 5. In order for the sum of 136 to be divisible by 5, the unit number of this number must be 1 or 6, and only 26 out of 7 bags of rice are 6;

So, the weight of the bag of rice bought by A is 26 kilograms.