let's write it as fractions:
[tex] \frac{2}{5} \frac{5}{7} \frac{1}{2} [/tex]
we can simplify whatever is both in the nominator and the denominator: so fist, 5:
[tex] \frac{2}{1} \frac{1}{7} \frac{1}{2} [/tex]
[tex] \frac{2}{7}\frac{1}{2} [/tex]
now, 2:
[tex] \frac{1}{7}\frac{1}{1}[/tex]
so:
[tex] \frac{1}{7}[/tex]
this is the answer!