No. Of cases that the number on the page is divisible by 10 = {10,20,30,.....200}
Now it is an A.P.
To find n use formula of last term i.e. \(a_n = a + (n-1) d\)
200 = 10 + (n-1) 10
So n = 20
Probability = possible outcome/ Total outcome
= 20/200 = 1/10