a)
well, she has a total of 70 + 30 tiles, now she's not going to used all of them, just some.
the dark area is 8x5, so is 40 ft², and the light gray area is 8x4 or 32 ft². The total area is then 40 + 32, or 72 ft².
b)
the dark area is an 8x5 rectangle, so is 8*5 or 40, so she'll need 40 dark tiles, but she has 70, so that's fine.
the light gray area is a 8x4 rectangle, 8*4 is 32, but she only has 30 light gray tiles, so she's short 2 light gray tiles.