20. Using arithmetic operation signs, write the largest natural number with two twos.
$\triangle$ Let's write down all natural numbers using two twos. There are not many of them:

$$
2+2=4, \quad 2 \cdot 2=4, \quad 2^{2}=4,22
$$

The largest of these is the number 22. Interestingly, for its representation, no arithmetic operation symbols were needed at all.

Answer: 22.