PA HELP PO!! NEED KO NA PO TALAGA ITO

18444kh+7+15*5+6+1978-32e「12-5426中-1)」​

Select 10 pairs of numbers of relatively prime from these numbers 13, 36, 78, 21, 34, 67, 15, 27, 69, 25 and write thwm as ratios

Hey guys why does 1/x is equals to[tex]x {}^{ - 1 }[/tex]what rule is it ? ? - freshman engineering here​