Answer: 30 feet per second
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x = diameter, y = speed of water
y varies inversely with respect to x, so, y = k/x for some constant k
If x = 0.6 and y = 5 pair up, then y = k/x turns into 5 = k/0.6 which solves to k = 3 when you multiply both sides by 0.6
So the equation y = k/x turns into y = 3/x
Now plug in the diameter x = 0.1 to find the speed to be...
y = 3/x
y = 3/0.1
y = 30 feet per second