To add #1/6#, #2/3# and #5/12#, we first need to find the Lease Common Multiple (LCM) of the three denominators i.e. #6, 3#, and #12#.
As #6=2xx3#, #3=3# and #12=2xx2xx3=2^2xx3#
LCM of #6, 3#, and #12# is #2xx2xx3=12# (as it includes the highest exponent of each prime factor - for details see here ).
Now let us multiply numerator and denominator of each fraction so that denominator of each fraction is #12# i.e.
#1/6=(1xx2)/(6xx2)=2/12#, #2/3=(2xx4)/(3xx4)=8/12# and #5/12# already has #123 as denominator.
So #1/6+2/3+5/12=2/12+8/12+5/12=(2+8+5)/12=15/12#
As numerator and denominator of sum #15/12# can be divided by #3#
#15/12=(15-:3)/(12-:3)=5/4= 1 1/4#
