An ant sits on an infinite grid at (0, 0) as shown below. There is also foodindicated by a dot at (5, 3). The ant is not aware yet of the oblique line(that’s Broadway of the ant city), but she will be in part (e).(a) Assume that the ant senses where the food is and will start moving to-wards it by only making UP and RIGHT moves. How many possible pathsare there that will bring the ant to the food? Hint: think of a bijection fromthe set of paths to some special set of binary words (what is that set?).2
Question
An ant sits on an infinite grid at (0, 0) as shown below. There is also foodindicated by a dot at (5, 3). The ant is not aware yet of the oblique line(that’s Broadway of the ant city), but she will be in part (e).(a) Assume that the ant senses where the food is and will start moving to-wards it by only making UP and RIGHT moves. How many possible pathsare there that will bring the ant to the food? Hint: think of a bijection fromthe set of paths to some special set of binary words (what is that set?).2
Solution
The problem can be solved using the concept of permutations and combinations. The ant needs to make a total of 5 moves to the right (R) and 3 moves up (U) to reach the food. This is a total of 8 moves.
We can think of each path the ant can take as a sequence of these 8 moves. For example, one possible path could be RRRRRUUU (first move 5 steps to the right, then move 3 steps up).
The number of such paths is the number of ways to arrange 5 R's and 3 U's, which is a combinatorial problem. The formula for combinations is:
C(n, r) = n! / [r!(n-r)!]
where n is the total number of items, r is the number of items to choose, and '!' denotes factorial.
In this case, n = 8 (total number of moves) and r = 5 (number of right moves), so the number of paths is:
C(8, 5) = 8! / [5!(8-5)!] = 56
So, there are 56 possible paths the ant can take to reach the food.
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