26. Five mark exercise problem VII

A man whose eye-level is $2 \ m$ above the ground wishes to find the height of a tree. He places a mirror horizontally on the ground $20 \ m$ from the tree and finds that if he stands at a point $C$ which is $4 \ m$ from the mirror $B$, he can see the reflection of the top of the tree. How height is the tree?

 

The height of the tree $=$  $m$