Time required for making a print at a distance of 40 cm from a 60 watt lamp is 12.8 second. If the distance is decreased to 25 cm, then time required in making the similar print will be

5 sec

If prints are similar,

$E_1t_1 = E_2t_2 ⇒ \frac{I_1}{r1^2}t_1 =\frac{I_2}{r2^2}xt_2$

But $I_1 = I_2$

$∴ t_2 =\frac{r2^2}{r1^2}xt_1 ⇒ t_2 = 5 sec$