math...help

Let y be the present age of Peter's sister.
Then, (y+6) will be the present age of Peter.

(y+6)-4=3(y-4)

y+6-4=3y-12

14=2y

y=7

So, the present age of Peter's sister is 7 years old.

2007-12-29 13:27:02 補充：
Because "4 year age Peter was three times as old as his sister ". I assume you have made a typing mistake that it is "4 year ago". So, 4 year ago, Peter's age was y+6-4. Similarly, 4 year ago, his sister's age was y-4. Three times means "x3".

(y+6)-4=3(y-4)
(y+6)-4=3y-3*4
(y+6)-4=3y-12
y+6=3y-16
4y=10
y=2.5
peter sister age is 2.5+4
=6.5years old

彼得比他的姐妹, 4 年年齡彼得三倍老像他的姐妹6 年年紀。什麼是彼得的姐妹的年齡?
4x3+6=18
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