Question

Is 3.142857 a rational number

Answer 1

If you're talking about just the decimal, then yes it is.
If you're talking about 22/7, then it still is.
But if you're talking about pi, then it isn't rational.

Answer 2

A rational number is any number that can be expressed as the ratio between two integers; i.e. by division or as a fraction. In this case:
$3.142857=\frac{3,142,857}{1,000,000}$

Be careful here. $\frac{3,142,857}{1,000,000}$ is very close to $\frac{22}{7}$ (a common approximation for pi) but $\frac{22}{7}$ does not terminate, it repeats while 3.142857 does terminate. Both are rational but they are not equal.