If the area of circle P is 1,600 units squared, in units what is the diameter?

45.2

Let's see how critical  might have gotten this

1600  = pi * r^2      divide both sides by pi

1600 / pi   = r^2

Take the square root of both sides

sqrt (1600 / pi )  = r   = 22.56

And the diameter is twice this ≈ 45.2