A boat costs x dollars, and this cost is to be shared equally by a group of people. In terms of x, how many dollars less will each person contribute if there are 4 people in the group instead of 3 ?
If there are three people each pays $\displaystyle \$\frac{x}{3}$.
If there are four people each pays $\displaystyle \$\frac{x}{4}$.
The difference is
$\displaystyle \$\frac{x}{3} - \$\frac{x}{4}$
$\displaystyle = \$\frac{4x}{12} - \$\frac{3x}{12}$
$\displaystyle = \$\frac{x}{12}$.