DL has often corrected me for not putting the -on- in the last place of the sentence, but this time rejected: -Montako minuuttia metro myöhässä on?-. Does -on- not have to be in the last place for -Montako-?
Your answer would've been correct. It sounds a bit strange, but grammatically correct.
In many exercises, "aikaa" is required to denote a length of time. Why is "Montako minuuttia aikaa metro on myöhässä?" incorrect?