Question 384111
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Hi,         
Note:  It is necessary to find a common denominator to combine fractions.
As I understand the Question: the answer You refer to is incorrect.
 {{{ x + 1/12 = 3/20}}}
 {{{ x = 3/20 -1/12 }}}  common denominator being 60
 3/20 = 9/60  and 1/12 = 5/60
 {{{ x = 9/60 - 5/60 }}} 
    x = 4/60 = 1/15

CHECKING our Answer
1/15 + 1/12 = 4/60 + 5/60 = 9/60 = 3/20