The answer, surprisingly, is 6.28 meters, or 0.000016%. The additional zipper needed is proportional to the diameter increase, which is 2 meters (one on each side). 2 meters times pi is 6.28 meters.

In equation format:

Let *D* be the diameter of Earth at the equator in meters.

It is highly counterintuitive that so little is needed for such a seemingly big job. The three choices provided were there to mislead you. Even though the range of choices I offered seemed reasonable (1%/10%/100%), sometimes the real answer is closer to “none of the above”. Always consider alternatives – real life issues often have solutions that are “none of the above”. Indeed, real life issues often don’t have neat solutions at all.