How many ordered pairs of positive integers $(a, b)$ are there such that a right triangle with legs of length $a, b$ has an area of $p$, where $p$ is a prime number less than $100$?

[i]Proposed by Joshua Hsieh[/i]
To find the number of ordered pairs of positive integers \((a, b)\) such that a right triangle with legs of length \(a\) and \(b\) has an area of \(p\), where \(p\) is a prime number less than 100, we start by using the formula for the area of a right triangle:

\[
\text{Area} = \frac{1}{2}ab
\]

Given that the area is \(p\), we have:

\[
\frac{1}{2}ab = p \implies ab = 2p
\]

Since \(p\) is a prime number, \(2p\) is an even number. We need to find the number of ordered pairs \((a, b)\) such that \(ab = 2p\).

1. **Identify the prime numbers less than 100:**
   The prime numbers less than 100 are:
   \[
   2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
   \]
   There are 25 prime numbers less than 100.

2. **Determine the number of pairs \((a, b)\) for each prime \(p\):**
   For each prime \(p\), we need to find the pairs \((a, b)\) such that \(ab = 2p\). Since \(a\) and \(b\) are positive integers, we consider the factors of \(2p\).

   - If \(a = 1\), then \(b = 2p\).
   - If \(a = 2\), then \(b = p\).

   These are the only two pairs for each prime \(p\) because \(p\) is prime and \(2p\) has only four factors: \(1, 2, p, 2p\).

3. **Count the pairs for all primes:**
   Since there are 25 primes and each prime \(p\) gives us 2 pairs \((a, b)\), we have:
   \[
   25 \times 2 = 50
   \]

4. **Consider the ordered pairs:**
   Each pair \((a, b)\) is distinct from \((b, a)\) unless \(a = b\). However, since \(a\) and \(b\) are factors of \(2p\) and \(p\) is prime, \(a\) and \(b\) cannot be equal. Therefore, each pair is unique.

Thus, the total number of ordered pairs \((a, b)\) is:

\[
\boxed{50}
\]