90. Find the number of all trees planted at five-foot intervals from each other on a rectangular plot of land, the sides of which are 120 ft. and 70 ft.
90. The solution of Epaphroditus. $\frac{120}{5}=24$ and $\frac{70}{5}=14$, therefore, along the base of the plot, 25 trees are planted, and along the height, 15 trees, and in total $25 \cdot 15=375$ trees. The quotients 24 and 14 represent the number of intervals, hence the number of trees in each row is one more. Generally, if the dimensions of the plot are $a$ and $b$, and the interval is $d$, then the number of trees on the plot is:

$$
N=\left(\frac{a}{d}+1\right)\left(\frac{b}{d}+1\right)
$$