Itoni mbalamofipi ya imi ni tahi shuma zwithu

There are 20 circles in total.
12 of them are crossed out.
To find how many are left, we subtract the crossed-out circles from the total.

$$20 - 12 = \boxed{8}$$

The problem shows a sum of $17 + 23$.
First, add the ones digits: $7 + 3 = 10$. Write down 0 and carry over 1 to the tens place.
Next, add the tens digits: $1 + 2 + 1 (\text{carry-over}) = 4$.
The sum is 40.

$$\begin{array}{c} \quad 17 \\ + \quad 23 \\ \hline \quad \boxed{40} \\ \end{array}$$