"Math" puzzler on whitehousekids.gov

Factual Questions
21

8 @ 12: 12 = 2 * 2 * 3. 3 doesn’t divide 8 evenly, so you multiply 8 by 3 and add 12 / 3.
9 @ 5: 5 is a prime, and doesn’t divide 9 evenly, so you multiply 9 by 5 and add 5/5.
15 @ 24: 24 = 2 * 2 * 2 * 3. 2 * 2 * 2 doesn’t divide 15 evenly, so you multiply 15 by 8 and add 24 / 8.
10 @ 22: 22 = 2 * 11. 11 doesn’t divide 10 evenly, so you multiply 10 by 11 and add 22/11.

therefore,
10 @ 18: 18 = 2 * 3 * 3. 3 * 3 doesn’t divide 10 evenly, so you multiply 10 by 9 and add 18 / 9

22

Actually, let me put it in more mathematical terms:

a @ b = gcd(a,b) + a * b / gcd(a,b)

where gcd(x,y) is the greatest common divisor of x and y.

23

I thought about that one but missed it somehow - the White House kids are sharp !