Three milkshakes equals 30.

1 milkshake is 10.

1 milkshake plus 2 hamburgers equals 20.

1 Hamburg equals 5.

Then you should read the topic clearly.

1 hamburger and two double fries make 9.

1 double fries is 2.

The last formula, French fries are 1 bag, not double, and double French fries are 2 1 bag, which is 1.

The last formula is actually 5+ 1× 10.

So the result is 15.