Explanation

This is an arithmetic sequence where the difference between successive terms is always +10.

The difference between, for example, $a_{10}$ and $a_{11}$, is exactly 10, regardless of the actual values of the two terms. The difference between $a_{10}$ and $a_{12}$ is $10 + 10 = 20$, or $10 \times 2 = 20$, because there are two “steps,” or terms, to get from $a_{10}$ to $a_{12}$. Starting from a10, there is a sequence of 5 terms to get to $a_{15}$.

Therefore, the difference between $a_{10}$ and $a_{15}$ is $10 \times 5 = 50$.