From a train station, one train heads north, and another heads east. Some time later, the northbound train has traveled 27 kilometers, and the eastbound train has traveled 36 kilometers. How far apart are the two trains, measured in a straight line?

the eastbound train had travelled 12 miles
Explanation: using Pythagoras theorem, we know that
X^2 = a^2 + b^2
Where ^ stands for raised to power.
Let
a stand for the train going towards North and
b stands for the train going towards east.
X stands for the total distance between train a and b = 20 miles.
By Pythagoras rule
X^2 = a^2 + b^2
20^2 = 16^2 + b^2
400 = 256 + b^2 so that
b^2 = 400 - 256 = 144
b = √144
b = 12 miles.