If there are 48 tiles in each row then minus 48 - 6 white tiles = 42 purple tiles. And if there are 8 rows then 8 x 42 = 336. So Abigail needs 336 purple tiles.
.75 is 75% which is also 3/4 or three-quarters. They all mean the same thing. That means he makes 3 out of every 4 shots. If he shot 16 times he made 12 shots because: 16 X (3/4) = 12