Natural
Reasonable, Obvious, Usual, Common
You've seen how 当たり means success. If you've experienced success before, everything after that seems to be pretty obvious.
あたりまえ
The readings for this word are both kun'yomi readings. You've learned both of these readings by now so you should be able to read this on your own.
こんな当たり前のこと、どうして分からないの？
This is normal stuff, why can’t you get that?
でも、これが当たり前のことだとは思わないでくださいね。
Don't take it for granted though.
今日のような当たり前の日をすごせることに、かんしゃしなきゃいけませんよね。
Shouldn't we be thankful we're able to have an ordinary day like today.