Answers

2016-01-20T20:13:01+05:30
First we have to find the area of the circle and multiply its circumference by the height and add the both to get cardboard used for ONE pen holder.
∴(pi)(r²)
=(22/7)(5)
=110/7.
Now its circumference=(pi)(d)
=(22/7)(10)
=220/7.
It should be multiplied by height.
∴(220/7)(14)
=3080/7
Now Adding
(3080/7) + (110/7)
=3190/7.
now we have to multiply this by 42.
∴3190×42/7
=3190×6
=19140 cm
∴19140cm of cardboard was consumed.
1 5 1
Your process is right but i think your calculation is wrong
you took area of circle as pi r^2 but later multiplied only one r
well thats what happens when u r in a hurry
u r wrong as well 
can u pls specify where m  i wrong?
2016-01-20T20:13:03+05:30
No. of students = 42
Radius of holder = r = 5cm
Height of holder = h = 14cm
Now, 
Area of card board consumed = 42 x (curved surface area  of cylinder + area of base)
                                                 = 42 x (2 x  \pi  x r x h +  \pi                                                        x  r^{2}
                                                 = 42 x (2 x 22/7 x 5 x 14 + 22/7 x 5 x 5
                                                 = 42 x (2 x 22 x 5 x 2 + 22/7 x 5 x 5)
                                                 = 42 x (440 + 550/7)
                                                 = 42 x (440 x 7 +550)/7
                                                 = 6 x 3630
                                                 = 21780  cm^{2} ,, = Ans
1 1 1
no ur answer is wrong
the pen holder is not stuffed with cardboard that u r calculating volume
21780 cubic cm cant be the answer because it should be a unit of area 
I guess u r in dreams