A 1.5 m tall boy is walking away from the base of a lamp post which is 12 m high, at the speed of 2.5 m/sec. Find the…

Let AB be the lamp post and CD be the boy $1.5$ m tall.
For correct figure
Let the length of shadow be $x$ m
Speed of boy = $2.5$ m/sec
$\therefore$ Distance covered in $3$ seconds = $7.5$ m
Now, $\triangle ABE \sim \triangle CDE$
$\frac{CD}{AB} = \frac{DE}{BE}$
$\frac{1.5}{12} = \frac{x}{7.5+x}$
Solving, we get $x = \frac{15}{14}$ or $1.07$ approx.
Hence length of shadow is $1.07$ m