Antwoord:
#x = 40 #
Uitleg:
# => 1/2 (180 - x) + (90 - x) = 120 #
# => 180/2 - x / 2 + 90 - x = 120 #
# => 90 - x / 2 + 90 - x = 120 #
# => 90 + 90 - x - x / 2 = 120 #
# => 180 - (3x) / 2 = 120 #
# => (3x) / 2 = 180 - 120 #
# => (3x) / 2 = 60 #
# => x = 2/3 × 60 #
#=> 40#
