Answer:
A tree of 60 feet casts the shadow of 100 feet, at the same time, what is the length of the shadow of a boy who is 3 feet tall ?
Solution :
Let b represents the length ( in ft ) of the shadow of the boy,
∵ Height of the boy = 3 ft,
So, the ratio of the height of boy and his shadow = 
Now, the tree of 60 ft long casts 100 ft shadow,
So, the ratio of height of tree and its shadow = 
At the same time,

Which is the required equation.