1. The main formula related to menstruation cycle calculation is the menstrual cycle length, which is typically about 28 days but can range from 21 to 35 days. 2. Another key formula involves predicting ovulation. Ovulation usually occurs about 14 days before the start of the next menstrual period. Thus, Ovulation Day $$= \text{Cycle Length} - 14$$. 3. The fertile window is typically 5 days before ovulation and 1 day after. To calculate the fertile period: - Start of fertile window $$= \text{Ovulation Day} - 5$$ - End of fertile window $$= \text{Ovulation Day} + 1$$ 4. Sample question 1: If a woman has a 30-day menstrual cycle, on which day does she ovulate? - Solution: $$30 - 14 = 16$$, so ovulation occurs on day 16. 5. Sample question 2: If a woman’s cycle is 26 days, what is her fertile window? - Ovulation day $$= 26 - 14 = 12$$ - Fertile window $$= 12 - 5 = 7$$ to $$12 + 1 = 13$$, so days 7 to 13 are fertile. These are key menstrual cycle formulas relevant for BGCSE level.