A sign made of solid wood is in the shape of a pyramid. The base is a triangle with a base of 6 feet and a height of 4 feet. The height of the sign is 7 feet. The wood costs $3 per cubic foot. Find the cost of the sign.

The correct answer is: $84

Given:
a base of a pyramid b =  6 feet
a height of  a pyramid h = 4 feet
To find: 
Cost of the sign made of solid wood in the shape of a pyramid =?
Solution:
A sign is made up of solid wood in the shape of a pyramid.
The base of a  triangle is 6 feet and the height is 4 feet.
The height of the wooden sign is 7 feet.
Suppose the V, h, l, and b be the volume, height, base length, and base width of the pyramid.
The volume of a pyramid is given by the formula,

V =   6 4 7
V = 56 feet³
But The wood costs $3 per cubic foot.
Suppose T is the total cost of the sign. 
T = 56 3
T = $168 
So the correct option is b.

First, we can calculate the base area and then multiply it by height.