[piotrSikora]

0.004% is 0.00004, not 0.004, so the result is 50 billion galaxies.

[ccollins]

Geuis:

.004 / 100 == .00004 / 1

And

.004% = .00004

[stagas]

100 / 0.004 = 25,000

So we'd need 25,000 images like that one, to fill the sky.

25,000 * 200,000 = 5,000,000,000 galaxies

[geuis]

I don't think that's right. The author clearly states its 0.004% of the sky, not 0.00004%. Simply adding a percent sign doesn't change the number.

[iclelland]

Yes it does, actually: "per cent" means "per 100", so, divide by 100.

50% is 50/100, or 0.5 of the whole

0.004% of the sky is 0.004/100, or 0.00004 of the whole sky.

[prophetjohn]

It's right. 0.004% == 0.00004

Just like 100% is equal to 1. If the image were 100% of the sky, you would divide by 1, not 100.

[tadfisher]

Yes, and 0.002 cents == $0.002.