Hello, pashah!

Your answer to part (a) is incorrect . . .

. . and it seems to be a trick question (on a number of levels)

a. If one Helium (gas) tank can fill 16 nine-inch diameter spherical balloons,

then the same tank can fill how many 3-foot diameter balloons?

A nine-inch diameter means a -inch radius.

One nine-inch balloon holds: . in³ of helium.

Since the tank can fill 16 of these balloons,

. . the tank holds: . in³ of helium.

Now a 3-footdiameter means a 36-inch diameter . . . an 18-inch radius.

These larger balloons hold: . in³ of helium.

Hey! . . . the tank doesn't hold enough to fill even one 3-foot balloon.

Answer:zero

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Oh, sure . . . you could fill one balloon part way.

But then it wouldn't be athree-foot balloon, would it?