form1 Fibonacci sequence maths question

The 5th term =5
The 10th term = 55
The 15th term = 610

Their common factor is 5

Let f(n) be the n-th term of the Fibonacci sequence.

From the definition,
f(1) = 1
f(2) = 1
f(n) = f(n-2) + f(n-1) for all n > 2

So the first 15 Fibonacci sequence numbers are
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610

So
f(5) = 5
f(10) = 55
f(15) = 610

Their common factor is 5.