6. $\underbrace{2 \times 2 \times \ldots \times 2}_{20 \uparrow 2}-1$
The unit digit of the result is $\qquad$
【Answer】 5
Analysis: $2 \div 10$ remainder is $2, (2 \times 2) \div 10$ remainder is $4, (2 \times 2 \times 2) \div 10$ remainder is $8, (2 \times 2 \times 2 \times 2) \div 10$ remainder is 6, that is, 4 twos multiplied together form one cycle, $20 \div 4=5$, so $\underbrace{2 \times 2 \times \ldots \times 2}_{20 \uparrow 2}$ remainder is 6, $\underbrace{2 \times 2 \times \ldots \times 2}_{20 \uparrow 2}-1$
$\div 10$ remainder is 5