Question

How many rulers of 12-inches are needed to measure a door awnser

Answer 1

the dimensions of a door are variable, for effects of this problem we will assume a door height of 210 cm and a door width of 80 cm

Step 1
convert the dimensions of the door in cm to inches
we know that
1 in-------------> 2.54 cm
X----------------> 210 cm
X=210/2.54=82.67 in (height)

1 in-------------> 2.54 cm
X----------------> 80 cm
X=80/2.54=31.50 in (width)

the dimensions of a door are 31.50 in x 82.67 in

Step 2
calculate the amount of rules of 12-inches necessary to measure the height of the door
82.67 in (height)
if one rule-------------> measure 12 in
X----------------------> 82.67 in
X=82.67/12=6.8-----------> 7 rules

Step 3
calculate the amount of rules of 12-inches necessary to measure the width of the door
31.50 in (width)
if one rule-------------> measure 12 in
X----------------------> 31.50 in
X=31.50/12=6.8-----------> 2.62 ------------> 3 rules

the answer is 
to measure a door are needed about 7 rules of 12-inches for the height and about 3 rules of 12-inches for the width