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Can you replace the question mark with a number?

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

Correct answers: 39

The first user who solved this task is Nasrin 24 T.

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What are the three words tha...

- What are the three words that are most often found in the inscriptions in the world?
- "I love you".
- Not really. Those three words are "Made in China"!

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Catalysis

In 1823, the effect of platinum as a catalyst was investigated by Johann Wolfgang Döbereiner, who found that hydrogen combined with air in the presence of platinum powder to form water, in a reacttion so vigorous that the filter paper holding the powder was charred. (It was later that the term catalysis was coined by Berzelius, in 1835.) Within a few days, on 3 Aug 1823, he invented the Döbereiner lighter process using the effect he had observed. He ignited a thin jet of hydrogen by passing it over spongy platinum in air. He designed a jar-type lighter using this method. Within three months, news of his discovery had been reported in some dozen European science journals. His lighter design was taken up by manufacturers to replace starting a fire with flint and tinder, and was in production for several decades.«

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