Question 38279
Make a right triangle diagram, with one leg going north (y) and one going east (x).  Label the hypotenuse z.
In general D = RT, distance = rate*time, so we can write
y = 20t and x = 30t
Now from the Pythagorean Theorem, we know
z^2 = x^2 + y^2
So let us substitute
z^2 = (30t)^2 + (20t)^2
z^2 = 900t^2 + 400t^2
z^2 = 1300t^2
z = sqrt(1300t^2)
z = 10t*sqrt(13)
and we're done with part a...
In part b, t = 10, so 
z = 10*10*sqrt(13)
z = 100*sqrt(13)
z = 360.6 mi