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Statistic probability help!?

So here's the Q:
A number is picked at random from the first 100 positive whole numbers (starting at 1). What is the probability that the number is divisible by 5, given that it is even?
Give your answer as an exact decimal or as a fraction in its lowest terms in the form p/q.
I am really struggling with probability and it's god damn f**king frustrating. I am so sick of it.
I cannot follow the formulae, I just cant, I dont understand how to use it.
So here's my thought process:
probability its divisble by 5 = 20/100 = 1/5
probability its even = 50/100 = 1/2
so probability even and divisible = 10/100 = 1/10
now that all the odd numbers have been eliminated, we only have 50 values left to pick from.
The reason for this is that it is DEFINITELY certain to be even, so with only 50 numbers left, out probability of it being divisible by 5 should double and become 1/5.
I am already tying myself in knots. I simply cannot understand this topic and I need help asap. Can someone please help me!?

Reply 1

PhysicsMathsTut

You can use the formula:
P(A|B)=P(AandB)/P(B)
In this case it becomes P(even and divisible by 5)/P(even)

Or you can say since you know it's even, you are choosing out of 50 (q=50). Out of those 50, how many are divisible by 5 (that becomes p)? The probability is then p/q

You should be getting 1/5