To add #1/3+5/12+4/5#, firs convert all denominators to Lease Common Denominator (LCD), which is nothing but Least common multiple of #3#, #12# and #5#. As #3# is a factor of #12#, their LCD is #12# and LCD of #12# and #5# is #12xx5=60# (as nothing is common between them. Hence LCD of #3#, #12# and #5# is #60#.
Hence, converting each denominator of #1/3+5/12+4/5# to LCD,
#1/3+5/12+4/5# = #(1xx20)/(3xx20)+(5xx5)/(12xx5)+(4xx12)/(5xx12)#
= #20/60+25/60+48/60#
= #(20+25+48)/60=93/60=(31cancel93)/(20cancel60)=31/20=1 11/20#
(We have divided #93# and #60# by #3# here to simplify.)