Your brain, of course.
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It depends on your computational skills.
Break it up into two separate problems. First 4X25=100, which is easy enough. Then 27X100=2700.
The term that describes numbers that are easy to compute mentally is "round numbers." Round numbers are whole numbers that end in zero or five, making them simpler to work with in mental calculations. These numbers are often used in estimation and quick arithmetic tasks due to their ease of manipulation.
dont know about associative property but this one is easy in your head. 4x25=100x27=2700
No, but if you practice you can learn to do it and it gets easier. Start of by learning (by rote) your times tables and when adding up do so in 10s.