How many 15cm X 59.5cm tiles do you need to cover 428sq feet and can yo show me how to convert?

There are 2.54 centimeters in one inch. First, I would find the square footage of each tile.

(15/2.54)*(59.5/2.54)=(5.906*23.425)=138.348 square inches per tile

Now, we would divide that number by 144 (the number of square inches in a square foot) to find the square footage of each tile (138.348/144=.96sqft/tile)

Now divide your total you want to cover (428sqft) by your sqft/tile (.96sqft) and you get 445.83, so, 446 tiles to cover the area desired.

If you're laying it, I would figure in 10% for waste, making your total 491 tiles.

...and that is without grout joints :)