Find the area of the cardboard required to make a closed box of length 25 cm,

Length of the box $=25 \mathrm{~cm}$

Width of the box $=0.5 \mathrm{~m}$

$=0.5 \times 100 \mathrm{~cm}(\because 1 \mathrm{~m}=100 \mathrm{~cm})$

$=50 \mathrm{~cm}$

Height of the box $=15 \mathrm{~cm}$

$\therefore$ Surface are $a$ of the box $=2 \times($ length $\times$ breadth $+$ breadth $\times$ height $+$ length $\times$ height $)$

$=2 \times(25 \times 50+50 \times 15+25 \times 15)$

$=2 \times(1250+750+375)$

$=4750 \mathrm{~cm}^{2}$