If Valerie has a calculator, she can just calculate that sqrt(500) ~~ 22.36√500≈22.36, and order the least whole number greater than that, meaning she should order 23"ft"^323ft3 of cement.
If Valerie does not have a calculator, she can work out some squares of whole numbers to find out that
22^2 = 484 < 500 < 529 = 23^2222=484<500<529=232
As 22^2 < 500 < 23^2222<500<232, that means 22 < sqrt(500) < 2322<√500<23. Thus, without calculating the actual value of sqrt(500)√500, she can still tell that 2323 is the least whole number which is greater than sqrt(500)√500, and then order 23"ft"^323ft3 of cement.
