A person walks $15\text{ meters}$ east, then turns left and walks $10\text{ meters}$, then turns left again and walks $15\text{ meters}$. How far is he from the starting point?

Let the starting point be $(0,0)$ on a Cartesian coordinate plane.
1. Walks $15\text{ meters}$ east: The person moves to $(15,0)$.
2. Turns left (facing North) and walks $10\text{ meters}$: The person moves to $(15,10)$.
3. Turns left again (facing West) and walks $15\text{ meters}$: The person moves to:

$$(15-15,10)=(0,10)$$

4. The distance from the starting point $(0,0)$ to the final position $(0,10)$ is:

$$\sqrt{(0-0{)}^2+(10-0{)}^2}=10\text{ meters}$$