What is $0.1 \div 0.004$?
To divide decimals, I can first convert both numbers to fractions with powers of 10 as denominators. I can write $0.1$ as $\frac{1}{10}$ and $0.004$ as $\frac{4}{1000}$. Then, to divide fractions, I can multiply by the reciprocal of the divisor. So I have $\frac{1}{10} \div \frac{4}{1000} = \frac{1}{10} \times \frac{1000}{4}$. To simplify this product, I can cancel out the common factors of 10 in the numerator and denominator. I get $\frac{1}{10} \times \frac{1000}{4} = \frac{1}{1} \times \frac{100}{4}$. Now I can do the division of 100 by 4, which is 25. So the final answer is 25. 