VTMO 2015 — Grade 8 — final Question 5

Tanker $B$ starts $100 \,\mathrm{km}$ north of cargo ship $A$. A sails northwest at $20 \frac{\,\mathrm{km}}{h}$ and $B$ southeast at $30 \frac{\,\mathrm{km}}{h}$. How far has A traveled when they are closest?