No, if the diameter of the pipe is .6 m then its radius is .6/2= .3 m.
You should have 5/(pi* .3* .3)= 17.7 m/s.
Yes, this is exactly the same thing: diameter= 2*radius so (diameter)^2= 4*radius. Canceling the "4" in the numerator and denominator gives exactly what you have above.
Although you have the correct answer, there is a typo on the left: "pi" should not be squared.
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