There are 914.4 millimeters in a yard. There are 3 feet in a yard.
It means
1yard=914.4 millimeters , 1 yard= 3 feet1yard=914.4millimeters,1yard=3feet
So we have
914.4 millimeters = 3 feet914.4millimeters=3feet
Dividing both sides by 3
\frac{914.4}{3} millimeters = 1 feet3914.4millimeters=1feet
1 foot = 304.8 millimeters1foot=304.8millimeters
