We can put this information into two equations:
7 booklets = 140 stamps
3 booklets = 60 stamps
Let us then focus on one equation to find the relationship:
7 booklets = 140 stamps
If we put that into a fraction, we get

or

.
This means that for every booklet, Ashley gets 20 stamps; or rather, each booklet has 20 stamps.
We can then see that this relationship holds true for the second equation as well:

or

.
To write a general equation for this relationship using
p for the number of postage stamps, and
b for the number of booklets:
p = 20b
Where
b is equal to
p.