12/84 in unit fraction

1/7 is the unit fraction or the simplest form of the given 12/84

Look into:
Equivalent fractions are fractions having the same value even if they look different. Look different means they may not have the same denominator or numerator but, when you multiply the top (numerator) and the bottom (denominator) by the same integer, the fraction keeps its value.  
 Equivalent fractions can be simplified and written as the same fraction for example, 18/27 = 6/9 = 2/3.   

Example: 
Two equivalent fractions of 8/24.   
8/ 24 can also be 1/3. Why? How did it happen?   

Simply, get the GCF, the Greatest Common Factor or  the GCD, Greatest common Divisor of both numerator and denominator. 
8 = 1, 2, 4, 8 
24 = 1, 2, 3, 4, 6, 8, 12, 24 
8 is the greatest common factor of 8 and 24 so,
 _8   ÷  8_  = _1_  
  24      8          3   

8/24 is also equivalent to 2/6.  Thus;
8/24=2/6=1/3