The total cost of the white tiles is $12. We have 8 white tiles total. Divide the two values
12/8 = 3/2 = 1.50
The unit cost per white tile is $1.50
In other words, each white tile costs 1.50 dollars
---------------------
Another way to look at it using ratios is to say
12 dollars: 8 white tiles
Then divide both sides by 8 so that the "8 white tiles" turns into "1 white tile"
12/8 dollars: 8/8 white tiles
1.50 dollars: 1 white tile
Though this is effectively very similar to the prior section above.