To determine how many ways Pashmina can choose 2 apps from her 13 favorite apps, we can use the combination formula:
\[ C(n, k) = \frac{n!}{k!(n-k)!} \]
where:
- \( n \) is the total number of items to choose from (in this case, 13 apps),
- \( k \) is the number of items to choose (in this case, 2 apps), and
- \( ! \) denotes factorial, which is the product of all positive integers up to that number.
Plugging in the values:
\[ C(13, 2) = \frac{13!}{2!(13-2)!} = \frac{13!}{2! \cdot 11!} \]
We can simplify this as follows:
\[ C(13, 2) = \frac{13 \times 12}{2 \times 1} \]
Calculating this:
\[ C(13, 2) = \frac{156}{2} = 78 \]
So, the number of ways Pashmina can choose 2 of her favorite apps from the 13 on her phone is 78.