David pays $72.50 for 50 copybooks. How much was a copybook?

This is a unit rate problem, since 50 copybooks cost $72.50, we can set up the relationship below:
$72.5/(50 copybooks) = x/(1 copybook)
To get the cost for one copybook, we see that you have to divide both the numerator and denominator of the fraction on the left by 50.
$(72.5/50)/(50 copybooks/50) = x/(1 copybook)
= $1.45/(1 copybook)
A copybook costs $1.45.