It is far quicker to write
'c + t = 3' (5 characters)
than it is to write
'one cup of coffee and one cup of tea must be £3' (36 characters)
over 7 times as quick in fact!
I agree that in a proper mathematical proof you should define each variable, but in a simple example like this it should be quite obvious what 'c' and 't' stand for and seems a bit harsh to mark someone wrong for neglecting to formally define them.
If you want to be so strict about it I could just as well ask why 'one cup of coffee and one cup of tea must be £3' as you just stated. You've made a bit of a logical jump there haven't you in assuming that we all understand that its because £12 divided by 4 equals £3? Is that any different in me assuming that everyone understands what 'c' and 't' stand for in my shorthand notation?