個體經濟學題目 - 經濟

Izaak likes to eat pizza and to fish. the more fishing he
does the happier he is, up to 8 hours a day. If he fishes
longer than 8 hours he gets a sore back and is less happy
than if he hadn't fished at all. For y less than or equal
to 8, his utility function is U(x, y)=x+6y, where x is
money spend on pizza and y is hours per day spent fishing.
His income is $45 a day and he has no expenses other than
pizza. The Bureau of Fisheries has just decided to allow
people without fishing licenses to fish 3 hours a day. But
if you buy a fishing license, you can fish as many hours
as you wish. How much is Izaak willing pay for a license?

a.$33
b.$30
c.$48
d.$28
e.$0

我先寫上我以前學姐的的筆記
她的作法先是把x(income)=45和y(time spend on fishing)=3
代入utility function→U=45+6*3=63
到目前為止我還看得懂她在寫甚麼
不過接下來的式子就是45-m+6*8=63 然後m=30答案就出來了
有哪位高手可以解釋為什麼第二個式子可以這樣算嗎?
還是她省略了甚麼步驟?
感謝~<(_ _)>

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