Two circles of radii $8\text{cm}$ and $17\text{cm}$ are externally tangent to each other. Find the distance between their centres.

When two circles touch each other externally, the shortest distance between their centres ($d$) is equal to the sum of their radii ($r_1$ and $r_2$):
$$d=r_1+r_2$$

Given:

* $r_1=8\text{cm}$
* $r_2=17\text{cm}$

$$\text{Distance}=8+17=25\text{cm}$$