This seems to be a minor error, the code actually skips over the ++; operator entirely to get the desired result you can re-write this code with a simple change.
myNumber = 1;
myVar = (myNumber * 10 + 1);
This would yield the desired result. You can also place "++" on the Pre side instead of the Post side of the variable. (shown below)
myVar = (++myNumber * 10 + 1);
However this is interpreted as: 2 * 10 + 1 which is 21.
Hopefully this helps clear up any confusion.