Skip to content

Monotone grid paths hitting at least one power-up

Question

On a 6×66\times6 board a character starts at cell (1,1)(1,1) and must reach (6,6)(6,6), moving one cell at a time either down or to the right. Bonus items sit on cells (2,3)(2,3) and (4,6)(4,6). How many routes pass through at least one of the bonus cells?

Short answer

131131. Use inclusion-exclusion: routes through (2,3)(2,3) number (31)(73)=105\binom31\binom73=105, routes through (4,6)(4,6) number (83)⋅1=56\binom83\cdot1=56, and routes through both number 3⋅(52)⋅1=303\cdot\binom52\cdot1=30. So 105+56−30=131105+56-30=131.

Adapted from OpenQuant; question and answer rewritten by SelfMadeQuant.com.

See the hint and the full worked solution in the question bank (free account).

Open the worked solution

Lessons behind this question

Start with a free lesson: Probability Fundamentals is part of a free topic. Create a free account, no card needed, to read it in full with worked solutions.

Read it free