A boat heads due north across a river with a speed of $10\mathrm{.}0k\frac{m}{h}r$ relative to the water. The river has a speed of $5\mathrm{.}00k\frac{m}{h}$ due east, relative to the shore. What is the speed of the boat relative to the shore?

$14\mathrm{.}6k\frac{m}{h}r$

$12\mathrm{.}5k\frac{m}{h}r$

$11\mathrm{.}2k\frac{m}{h}r$

$15\mathrm{.}8k\frac{m}{h}r$

$10\mathrm{.}3k\frac{m}{h}r$