Keliling persegi panjang 30 cm, alasnya 9 cm, berapa lebarnya

K = 30
2(p+l) = 30
p+l = 30/2
p+l = 15
l = 15 - p
l = 15 - 9
l = 6 cm

9+9=18
30-18=12
lebar = 12 : 2 = 6 cm

semoga membantu