praying time

Question

During fasting month, the time of imsa' is usually 10 minutes or so before subuh. If someone usually keeps track of those times, he will approximately take 45 minutes before imsa' to end his sahur. If the imsa' time is 5:45 am, he would stop taking his sahur at 5.00 am. Create a project to calculate the time that someone will stop taking his sahur, given the imsa' time.

how do you execute that ?