唔識理題...唔明

設Ray10年前的為x歲,則父親10年前的為(x+20)歲
現年Ray為(x+10)歲,父親為(x+30)歲,因父年為Ray的兩倍,得
∵2(x+10)=x+30
2x+20=x+30
∴x=10
答:Ray10年前的為10歲

Let x be Ray's age 10 years ago and y be Ray's father's age 10 years ago.

Since 10 years ago,Ray's father's age is larger than Ray's age by 20,
y = x + 20

And now, his father's age is double of his age.
Therefore,
y + 10 = 2 ( x + 10 )
then simplify to y=2x + 10

y = x + 20 ----- (1)
y = 2x + 10 ----- (2)

Sub (1) into (2),
x + 20 = 2x + 10
x = 10

Therefore, Ray's age 10 years ago is 10.

let Ray's age 10 years ago as x

x+20+10=(x+10)2