A snapshot $${\text{1}}\frac{7}{8} \times 2\frac{1}{2}$$   is to be enlarged so that the longer dimension is 4. What will be the dimension of the shorter side ?

Let the dimension of the shorter side be x
More is the longer side, More is the shorter side (Direct proportion)
$$\eqalign{ & \therefore 2\frac{1}{2}:4::1\frac{7}{8}:x \cr & \Leftrightarrow \frac{5}{2}x = 4 \times \frac{{15}}{8} \cr & \Leftrightarrow x = \left( {\frac{{15}}{2} \times \frac{2}{5}} \right) \cr & \Leftrightarrow x = 3 \cr} $$