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

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