Jeff and Ursula each run $30 \mathrm{~km}$. Ursula runs at a constant speed of $10 \mathrm{~km} / \mathrm{h}$. Jeff also runs at a constant speed. If Jeff's time to complete the $30 \mathrm{~km}$ is 1 hour less than Ursula's time to complete the $30 \mathrm{~km}$, at what speed does Jeff run?
(A) $6 \mathrm{~km} / \mathrm{h}$
(B) $11 \mathrm{~km} / \mathrm{h}$
(C) $12 \mathrm{~km} / \mathrm{h}$
(D) $15 \mathrm{~km} / \mathrm{h}$
(E) $22.5 \mathrm{~km} / \mathrm{h}$
When Ursula runs $30 \mathrm{~km}$ at $10 \mathrm{~km} / \mathrm{h}$, it takes her $\frac{30 \mathrm{~km}}{10 \mathrm{~km} / \mathrm{h}}=3 \mathrm{~h}$.

This means that Jeff completes the same distance in $3 \mathrm{~h}-1 \mathrm{~h}=2 \mathrm{~h}$.

Therefore, Jeff's constant speed is $\frac{30 \mathrm{~km}}{2 \mathrm{~h}}=15 \mathrm{~km} / \mathrm{h}$.

ANSWER: (D)