Here, r = 3 cm, h = 10.5 cm

The penholders have only one base i.e., these are open at one end.

Total surface area of 1 penholder

= 2πrh + πr2 = πr (2h + r)

= 22 /7 × 3 (2 × 10.5 + 3) cm^{2}

= 22/ 7 × 3 × 24 cm^{2}

Total surface area of 35 penholders = 22/ 7 × 3 × 24 × 35 cm^{2} = 7920 cm^{2}

Hence, 7920 cm2 of cardboard is needed** Ans.**