Math Problem : GG.3 Find two cakes among three

When I bought three cakes from four cakes A, B, C, D, the total of weight was 1030 gram.

The weight of A is 40g, B is 60g, C is 60g, and D is 90g.

What are two cakes among three cakes I bought ?









Answer
A and D

Solution
The weight of B, C, D is a multiple of 30g, but 1030g is not a multiple of 30g. 

We can understand that A is included in three cakes. 

In addition, the weight of A, B, C is a multiple of 20g, but 1030g is not a multiple of 20g. 

We can understand that D is included in three cakes.