It seems like you cant rely on any computations and have to say it straight because “yesterday was 16” is also obvious but we are allowed to use it. In fact saying “tonight is 6 + 6 * 6” may be confusing to some and may cause errors (notice that i wrote your equation differently and a lot of people have a problem getting the right answer this way).
When we say the number it has to be obvious even to our chinuch age kids.