the altitude lines passes through thse pts..i.e

(2,3) & (5,5)
now obtain the equation of this line..

line is y= {2x+ 5 } / 3  ..

this can be written as.. y = {2(x+1) + 3}  / 3

so we want x+1 to be divisible by 3..

 

now it can be 2,5,8,11,14 ...(A.P = 3n-1)  where n  N.

now putting values of each in this equation..u get (y=a) type of line...

 

they also follow a pattern i.e ( 2n+1) where n  N..

 

so in given domain only..3,5,7,9 ..r included so probalility..is 2/5..

 

 

i think its fine..but better if a finer method is presented..