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a=bq+r

We say that q is the quotient and r the remainder obtained when b is divided into a.

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The underlined part, "b is divided into a", is it correct expression?

"when a is divided by b" seems to be absolutely correct, but "when b is divided into a" is pretty unfamiliar to me....
Comments  
Imagine you have 10 and you divide it into 5 different portions, you get 2 each. Got it?
Got it!!!