We should sail west.
Then find the distance of MN
So I'm n in the triangle
Just use Pythagorean theorem to find MN.
Suppose two ships are closest after x hours.
Then BN= 10x.
AM=20x,AN = 50- 10x 0 & lt; = x & lt=5
Mn 2 = am 2+(50-10x) 2 = (20 times) 2+(50-10x) 2.
MN= 10* quadratic root number (5x 2- 10x+25)
Let y = (5x 2- 10x+25)
=5[(x- 1)^2+4]
When x= 1, y has a minimum value.
So there is a minimum after 1 hour.