The question is

10 plus the quotient of a number and 15

A variable is something that does not have a fixed value (that is, it can have a value of 1, 2, 3 or any other number). It is often represented by a letter. The use of letters together with numbers in mathematics is algebra.

The most common letter used to represent a variable is x. So, I'll make use of x here.

We'll say, let x be the 'number' in your question.

Quote:

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10 plus the quotient of a number and 15

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What is the quotient of a number and 15? That would be the result of dividing x by 15, so,

Then, 10 plus that expression above?

Can you continue? :)