Negative one ninth. The reciprocal of nine is one ninth, so the reciprocal of one ninth is negative one ninth, so it is negative one ninth. Countdown refers to mathematically setting a number whose product of x is 1, recorded as one tenth of x, and the process is "multiplication inverse". All numbers except 0 have reciprocal, the numerator and denominator are inverted, two numbers whose product is 1 have reciprocal, and 0 has no reciprocal.