Instead consider what is the probability of NOT getting a 5. It is 5/6 if you do that n times you have (5/6)^n.

Hence, the probability of getting at least one 5 is 1-(5/6)^n

We want this to be expected, i.e,

1-(5/6)^n >= 1/2

Thus,

(5/6)^n <= 1/2

Find the first value of n which makes this true.