What's the rational number has - 0.875 as its decimal equivalent

Get rid of the decimal first.

-0.875*1000

-875/1000=-0.875

Simplify it since both denominator and numerator is divisible by 5

1/5 (-875/1000)

-175/200

Still divisible by 5

1/5 (-175/200)

-35/40

Keep going

-7/8

There, now it can't be more simplified.