Answer:
The height of the tower is 60 feet.
Step-by-step explanation:
Given,
The vertical pole 12 feet high casts a shadow 8 feet long on the ground.
So, the ratio of the height of pole and height of its shadow = 
Also, at the same time the tower casts a shadow of 40 feet long on the ground,
Let x be the height of the tower,
⇒ The ratio of the height of tower and height of its shadow = 
Since, at the same time,

( By cross multiplication )
( Dividing both sides by 8 )
Hence, the height of the tower is 60 feet.