一條方程式

設Dad歲數為x,則Mum歲數為x-3,Me的歲數是x*(1/5)
x*(1/5)=x-3-21
x/5=x-24
4x=120
x=30
即Dad歲數是30歲

i = Me's age
M = Mum's age
D = Dad's age

then:

i = (1/5) x D
可以變成......
D = 5i.......( 1 )

D = M+3
可以變成
M = D-3...( 2 )

i = M-21...( 3 )

將(3)擺落(1)o個度, 得到:
D = 5 x ( M - 21)....(4)

再將(2)擺落(4)度:
D = 5 x ( D-3 - 21)
D = 5 x ( D-24)
D = 5D - 120
D + 120 = 5D
120 = 4D
D = 30

所以Dad ge歲數係 30

(1)Me = Dad/5

(2)Dad-3 = Mum

(3)Mum - 21 = Me

so Mum -21 = Dad/5

(dad-3) -21 = Dad/5
dad -24 = dad/5
5dad - 120 = dad
5dad - dad = 120
4dad = 120
dad = 30

me = x
dad = y
mum = z
x = 5y --- (1)
x + 21 = z ---(2)
x = z - 21
5y = z -21
y = (z-21)/5