B is northeast, so ∠ BAC = 45,

In meaning, ∠ ACB = 90+ 15 = 105.

Then ∠ b = 180-45- 105 = 30,

BC/sin∠BAC=AC/sin∠B is determined by sine theorem,

BC=20x2sin45 /sin30 =40√2=56.56。