let x = the larger angle
$\large\frac{1}{2}x$ = the smaller angle
smaller angle$\large\frac{1}{2}x$ + larger angle (x) = 180 $\rightarrow$Linear Pair
$\large\frac{1}{2}x$ + x = 180
x + 2x = 360
3x = 360
x = 120 ------> larger angle
$\large\frac{1}{2}x$ = 60 -------> smaller angle