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Replace the question mark with a number

MATH PUZZLE: Can you replace the question mark with a number?

Correct answers: 129

The first user who solved this task is Thinh Ddh.

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A father and son are out shopp

A father and son are out shopping for Christmas presents for their family.

The son asks, "What present are my sister and I going to get?"

The dad answers, "I got you guys an iPad and iPod."

"Wow, thanks," the son replies, "What will you give mom?"

The dad says, "Your mom is getting an iRon."

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Achromatic lenses

In 1758, optician John Dolland, having created achromatic lenses, reported this to the Royal Society. Isaac Newton had described the blurring effect due to chromatic abberation, but could not solve the problem in simple lenses, which was important in the preparation of lenses for telescopes and microscopes. A concave face of a lens of flint glass (which has a greater color dispersion than crown glass) is cemented to a convex face of a crown glass lens. Thus the dispersion properties causing chromatic aberration are removed from both lenses simultaneously. Although Dollond patented the discovery of the achromatic lens in the mid-18th century, it is widely believed that he was not its inventor, but learned about it from lens maker George Bass.

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