$\Delta ABCand\Delta DEF$  are similar such that BC = 4 cm, EF = 5 cm and area of $\Delta ABC=64c{m}^2$ then the area of $\Delta DEF$ is

Give $\Delta ABC~\Delta DEF$

So ratio of their areas = ratio of the squares of correspo9nding sides

$\therefore \frac{area\left(\Delta ABC\right)}{area\left(\Delta DEF\right)}=\frac{B{C}^2}{E{F}^2}$

$\frac{64}{x}=\frac{4^2}{5^2}$

$\frac{64}{x}=\frac{16}{25}$

$16x=64\times 25$

$x=\frac{64\times 25}{16}$

$x=100c{m}^2$