Find 5 rational numbers between $\dfrac{2}{5}$ and $\dfrac{3}{5}$.

Find 5 rational numbers between $\dfrac{1}{6}$ and $\dfrac{2}{5}$.

Let a and b be two non-zero rational numbers such that $a + \dfrac{1}{b} = 0$. Without assigning any numerical values, determine whether ab is positive or negative. Justify your answer.

A rational number has a terminating decimal expansion whose last non-zero digit occurs in the 4th decimal place. Show that such a number can be written in the form $\dfrac{p}{10^4}$, where p is an integer not divisible by 10. Is it necessary that the denominator of this rational number, when written in the lowest form, is divisible by 2⁴ or 5⁴? Give reasons.