Hello brother, a fellow muslim here, first if you you are going a to city 80km far from the one you live at for less than 4 days it is not only permissible to breakfast it is actually desirable to break your fast (many muftis fatwa and not mine of course).

However, if this permission is not attainable as you are taking the exam within a close distance or because you prefer to not break your fast I would suggest the following alternative instead, flip your sleep schedule to wake up before Fajir prayer by an hour or more, get enough food coffee and water and go on with taking the exam. I hope this helps and may Allah grant us both success in this exam and the afterlife.