How many millimetres are there in fifteen centimetres

When solving a question like this, with two quantities and their different units (centimetres and millimetres in this case) you can begin by converting the values to the same units. this makes the calculations easier.

15 centimeters to millimeters = 15 x 100 ( because 1 cm has 100mm)

There are 1500 mm in 15 centimetres.

150 (in one centimeter there is 10 millimeters multiply 10 times 15 you get 150) 

8doors on the second floor