to find how many cm are in a ft, you have to use the method of unit rate:
so we know that 12 in. equals a ft. so,
1 ft
------
12 in.
and 2.25 cm equals an in. so,
1 in.
-------
2.54cm
then we put the rates together and get:
1ft 1 in.
---- X -------................we cancel out the in. units and get:
12 in. 2.54 cm
Solve and get 1 ft = 30.48 cm
