The cost of 4 pens and 4 pencils boxes is 1100.

Let the cost of a pen be ₹ x and the cost of a pencil box be ₹ y.

Then, by given condition,

$4 x+4 y=100 \Rightarrow x+y=25$ $\ldots($ (1)

and $\quad 3 x=y+15$

$\Rightarrow \quad 3 x-y=15 \quad$...(ii)

On adding Eqs. (i) and (ii), we get

$4 x=40$

$\Rightarrow \quad x=10$

By. substituting $x=10$, in Eq. (i) we get

$y=25-10=15$

Hence, the cost of a pen and a pencil box are ₹ 10 and ₹ 15, respectively.