Скачать или смотреть The 75 Trick [Math Trick Monday]

For today's Math Trick Monday, let's look at a cool use of both commutativity and associativity. When we multiply by 75, we can easily think of that as multiplying by 0.75 and then 100. Since 0.75 (or 75%) is the same as 3/4, rather than multiply by 75, we can divide the number by 4. If 4 goes into it evenly, we simply triple that and then write it down two zeroes after it. If there's a remainder, then we simply write that remainder times 75 itself. So, 75 for remainder 1, 150 for remainder 2 (adding the 1 to whatever we tripled for the first two digits of the product), and 225 for remainder 3 (adding 2 to the first two digits).

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This is a great trick if you participate in UIL Number Sense or are looking for other fast mental math techniques. Follow for more number sense tips.

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