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foto =
p = 30
l = 20

karton
p = 32 + x
l = 24

24/20 = (32 + x)/30
6/5 = (32 + ×)/30 -> kali silang

180 = 5(32 + x)
180 = 160 + 5x
20 = 5x
x = 4 cm