Boat travels west for 30minutes at the speed of 50kmph, so distance travelled in west direction = 50 * 1/2 = 25 km
Then both travels southeast for 1 hour @ 40kmph, so distance travelled in southeast direction = 40 * 1 = 40 km
The displacement from its origin can be calculated using law of cosine. And angle between them is 135 degree
Therefore displacement = sqrt( 25^2 + 40^2 - 2* 25* 40 *cos(135)) = 64.94 km (approx)