To figure out how much it will cost to fill the pool, you need to know how many gallons you need. For that, you need to know how many cubic feet of water can the pool hold. In other words, you first need to find the volume of the pool.
You're told the it's a circular pool, but you're also told it has a certain depth. So it can be treated as a cylinder.
The formula for the volume of a cylinder is V = π•r2•h
where: r is the radius and h is the height or depth
You're given the diameter: 20 feet. The radius would be half the diameter, so 10 feet.
The depth is given to you: 5 feet, so you have what you need to find the volume:
V = π•(10)2•5 = π•100•5 = 500π cubic feet [you can do the math in your calculator to get a numerical value for the volume]
Having found the volume in cubic feet, you have to find the number of gallons that would be needed. You're told one cubic foot of water = 7.48 gallons
So to find the # of gallons, you would multiply the volume in cubic feet by 7.48
# of gallons = 500π * 7.48 ----> Again, you can type it in your calculator to get a number for it
Now you have to find the cost. You're told water costs $3 per 1000 gallons. To find how much it would cost to fill the pool, you would take the # of gallons, multiply by 3 and DIVIDE by 1000:
Cost = ( # of gallons you found from earlier)*3/1000
Make sure to round that answer to the nearest dollar. You got this!