Question

a man drove his car northward,then turned left and drove 10 km and again turned left and drove 20 km. He found himself 10 km west of his starting point. How far did he drive northward initially?

Answer 1

Let us suppose he drive x km northward initially
now, drove 10 km left,
means now he is heading west
now drove left 20 km left,
means now he is heading south,
but he was found himself 10 km west from starting, 
that means he drove 20 km initially northward,
you can see the picture.

Answer 2

Draw the thing whatever you said . Eventually you will realize that his path represents a rectangle . Opposite sides of rectangles are equal . You will get 20 km