besar a° + b° + C° adalah

30 + 5 c = 180
5 c = 180 - 30 = 150
c = 150 : 5 = 30

sudut bcg = dcf = 180 - 5c = 180 - 5(30) = 30

sudut dcf + 2a + sudut bcg = 180
30 + 2a + 30 = 180
2a = 180 - 60 = 120
a = 120 : 2 = 60

sudut cab + 30 + sudut bca = 180
4b + 30 + 2a = 180
4b + 30 + 2 (60) = 180
4b + 30 + 120 = 180
4b = 180 - 150 = 30
b = 30 : 4
b = 7.5

jadi a + b + c = 60 + 7.5 + 30 = 97.5 derajat
(pembulatan 100 derajat)