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  

3rd option

Step-by-step explanation:

Cavalieri's principle states that two figures which have the same height and  same cross-sectional area have the same volume

the height of the cylinders are both 42

the diameter of both figures is 24 as the diameter of the second cylinder = 12 x 2 = 24

the radius of the cylinders are both equal to 12

the diameter is the straight line that passes through the centre of a circle and touches the two edges of the circle.

A radius is half of the diameter

thus the volumes should be equal