Question

A circular pond 20m in diameter is surrounded by a gravel path 5m wide. The path is to be replaced by a brick walk costing $30 per square meter. How much will the walk cost?

Answer 1

Here, check this out.

Answer 2

POND: 
diameter = 20m 
radius = diameter / 2 
radius = 20m / 2 = 10m 
area = 6.28 * (10m)^2 = 6.28 * 100m^2 
area = 628m^2 

POND and WALK: 
remember, the walk goes all the way around the pond, so there is 5m of wall on the west boundary and another 5m of wall on the east boundary. The diameter is increased by 10m, not just 5. 
diameter = 20m + 10m = 30m 
radius = diameter / 2 
radius = 30m / 2 = 15m 
area = 6.28 * (15m)^2 = 6.28 * 225m^2 
area = 1,413m^2 

WALK: 
The area of the walk is equal to the area of the pond AND walk minues the area of the pond alone. 
area = 1,413m^2 - 628m^2 
area = 785m^2