That depends how you want to "combine" them: add, subtract, multiply, etc.
The summands.
The LCM will be the product of the two numbers divided by 15.
They're 5 and 15.
3 and 15
x + y = 15 x - y = 7
The product of the prime numbers 3 and 5 is 15.
The two numbers are 5 and 15. 5 + 15 = 20. 15 - 5 = 10.
15 + 1 = 16, and 15 - 1 = 14, so the two numbers are 15 and 1.
12 and 3
3 & 15
3 and 12
12 and 3