So, to get to the a you want, count from the left, going right. When you get to the end of a row, go down to the left most tile in the next row. So, if I want the 23rd a, I count from left to right (5), then go down. Then I count again, and add 5. Until I get to the bottom row, then go 3. 3+20=23. So yeah. That's how you solve.
15/3=x
x+3=n
n/4=t
t+1+1=b
b+(0*3)+4=w
w-1=p
p-10=l
l+8=a
a+2=m
m=the number a you want to select
So, to get to the a you want, count from the left, going right. When you get to the end of a row, go down to the left most tile in the next row. So, if I want the 23rd a, I count from left to right (5), then go down. Then I count again, and add 5. Until I get to the bottom row, then go 3. 3+20=23. So yeah. That's how you solve.