On a given line, (on one side) there are a total of 180°
if one line in Problem #3 is bisected by a line, with one half X and the other 120°,
do 180° (the total) minus 120° which=60°
now the hard part, that line that bisected the first line is bisecting a line that is parallel to your second line, the one with <5 and <6
this means that the big angle formed in the first one with 120° is the same angle as in the second line, leaving <5 as 120°
which means <6 is 60°, like in the top part of the problem. You're basically flipping the top line upside down, I hope it helps.