So my little brother has a math problem, and one of them is this:

1/7+1/21=?

He insists on dividing the 21 with the 7, which is massively contrary to what I was taught, which is to find the least common multiple and add from there.

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Full list of problems:

1.) 1 1/2 + 1 1/2 =? (The answer is 3.)

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2.) 2 1/6 + 1/3 =?

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3.) 1 1/8 + 1/6 =?

4.) 1/7 + 1/21 =?

5.) 1/9 + 1 1/18 =?

Show your solution, aye?