Given: The length of the sides of a triangle are $15, 20, 25$ units.

Let $a=15, b=20, c=25$

As we know the circumradius of a triangle with length of sides $a, b, c$ units is given by, $\frac{abc}{\sqrt{\left(a+b+c\right)\left(a+b-c\right)\left(a+c-b\right)\left(b+c-a\right)}}$

$=\frac{15\times 20\times 25}{\sqrt{\left(15+20+25\right)\left(15+20-25\right)\left(15+25-20\right)\left(20+25-15\right)}}$

$=\frac{15\times 20\times 25}{\sqrt{60\times 10\times 20\times 30}}=12.5 \mathrm{units}$.