Luas permukaan (Lp) = 1.188 cm²

Tinggi tabung (t) = 20 cm

π = 3,14

Jari-jari tabung

Lp = 2πr (r + t)

1.188 = 2 × 22/7 × r (r + 20)

1.188 : 2 = 22/7 × r (r + 20)

594 = 22/7 r² + 440/7 r

594 × 7 = (22/7 × 7) r² + (440/7 × 7) r

4.158 = 22r² + 440r

4.158 : 22 = (22 : 22) r² + (440 : 22) r

189 = r² + 20r

189 - r² - 20r = 0

-189 + r² + 20r = 0

r² + 27r - 7r - 189 = 0

r (r + 27) - 7 (r + 27) = 0

(r + 27) (r - 7) = 0

r + 27 = 0

r - 7 = 0

r = -27

r = 7

Karena jari-jari tidak mungkin negatif, maka jari-jarinya 7 cm. Diameternya:

d = 2r

d = 2(7 cm)

d = 14 cm → option b