To determine the number of ways Pashmina can choose 2 apps from her 13 favorite apps, we can use the combination formula, which is given by:
\[ C(n, r) = \frac{n!}{r!(n-r)!} \]
where \( n \) is the total number of apps, \( r \) is the number of apps to choose, and \( ! \) denotes factorial. In this case, \( n = 13 \) and \( r = 2 \).
So the calculation is:
\[ C(13, 2) = \frac{13!}{2!(13-2)!} = \frac{13!}{2! \cdot 11!} \]
We can simplify this because \( 13! = 13 \times 12 \times 11! \):
\[ C(13, 2) = \frac{13 \times 12 \times 11!}{2! \times 11!} = \frac{13 \times 12}{2!} \]
Calculating \( 2! \):
\[ 2! = 2 \times 1 = 2 \]
Now substituting back into the equation:
\[ C(13, 2) = \frac{13 \times 12}{2} = \frac{156}{2} = 78 \]
Thus, the number of ways Pashmina can choose 2 of her favorite apps from the 13 on her phone is 78.