Hint: Transform 0.4 into fraction by multiplying and dividing by 10. Then divide the result by 2, or you can even multiply by $\dfrac{1}{2}$. This is a very easy and simple question to solve.
We have to find the value of, $0.4\div 2$ We can write the above equation as, $\dfrac{0.4}{2}...........(1)$ Now, consider the numerator part, that is, 0.4. First we will convert it into fractions. To make it into a fraction, let us divide and multiply the numerator of the given expression by 10, so we get $0.4=\left( \dfrac{0.4\times 10}{10} \right)............\left( 2 \right)$ What is $0.4\times 10$ now? Remember, multiplying by 10 just shifts decimal place to right. $\therefore 0.4\times 10=4.0...........\left( 3 \right)$ Now substituting value from equation (3) into equation (2), we get $0.4=\dfrac{4}{10}$ This is the fraction form of 0.4. Now, consider the expression (1), an substituting the fraction form of 0.4 in it,, we get $\dfrac{0.4}{2}=\dfrac{\dfrac{4}{10}}{2}$ We know that dividing by any number is nothing but multiplying by its inverse, so the above expression can be written as, $\dfrac{0.4}{2}=\dfrac{4}{10}\times \dfrac{1}{2}$ Now we know, $4=2\times 2$, so the above expression can be written as, $\dfrac{0.4}{2}=\dfrac{2\times 2}{10\times 2}$ Cancelling the like terms, we get $\dfrac{0.4}{2}=\dfrac{2}{10}$ Now we know dividing any number by 10, then the decimal shift one point to the left, so the above expression can be written as, $\dfrac{0.4}{2}=0.2$ Hence this is the required answer.
Note: Another method is to count the decimal places, for 0.4 that is one. Ignore decimal and divide 4 by 2, we get 2. Now, apply the one decimal place we get 0.2
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