Solution: Since one marker is mandatory, choose 1 additional marker from the remaining 3: $\dbinom31$. Then, select 2 sequences from 6: $\dbinom62$. Multiply these: $\dbinom62 \times \dbinom31 = 15 \times 3 = 45$. The total number of valid combinations is $\boxed45$.Question: A tech startup founder has 8 potential investment opportunities and wants to diversify by selecting 5, but two specific opportunities are mutually exclusive. How many valid investment portfolios can be formed? - go-checkin.com