A lot depends upon your interpretation of "opposite." Do you mean "reciprocal?"
If you give the number a, the reciprocal would be 1/a. Is that what you mean?
Or do you mean "the number with opposite sign?" If you give the number b, "the number with opposite sign" would be -b.
Consider both cases. Choose a rational number (such as 7/8). Find the reciprocal of this number. Find the number with opposite sign of this number.
