When fractions are divided by each other, you're really multiplying by the "multiplicative inverse". This is the fraction, but flipped.
[tex]\frac{8}{10} \div \frac{1}{5} = \frac{8}{10} \times \frac{5}{1} \\\\So\\\frac{8}{10} \times \frac{5}{1} = \frac{40}{10} = 4[/tex]
