先問2題,有既再補

Q21.
Betty's mother is three times as old as Betty this year. Three years later, the product of their ages will be 672.
Find the ages of Betty and her mother this year.

Q27(b)
Mr. Chan spent $500000 to buy car A two years ago, while Mr Lee spent $400000 to buy car B last year.
It is given that the value of each car decreases by r% every year.
The value of car A is $45000 higher than that of car B at present.
Find the value of r.

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+2題:

無計錯就係咁啦...



b:the age of Betty this year
m:the age of mother this year

3b=m
(b+3)(m+3)=672

(b+3)(3b+3)=672
3b^2+12b-663=0
b=13 or -17(rej.)
m=13x3=39

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500000(1-r%)^2=400000(1-r%)+45000
500000(1-r%)^2-400000(1-r%)-45000=0
(1-r%)=0.9 or -0.1
-r%=-0.1 or -1.1(rej.)
r%=0.1
r=10