well, let's bear in mind that about 2.54cm = 1 inch, and that 12 inches = 1 foot.
we simply use those ratios/proportions and flipped them upside-down as needed, depending on what we need cancelled.
[tex]\bf 15 \underline{ft}\cdot \cfrac{12 \underline{in}}{\underline{ft}}\cdot \cfrac{2.54 cm}{\underline{in}}\implies \cfrac{15\cdot 12\cdot 2.54 cm}{1}\implies 457.2 cm[/tex]
