幫我解下e條問題......

用substitution 可以0甘做既
let Amy originial has $x Bob has $y
then
x:y=7:4
4x=7y
y=(4/7)x ...(1)
After Amy gives $600 to Bob, the new ratio is 5:6
(x-600):(y+600)=5:6
6(x-600)=5(y+600)
6x-3600=5y+3000
6x=5y+6600...(2)
(1)代入(2)
6x=5(4/7)x+6600
(6-20/7)x=6600
(22/7)x=6600
x=2100
y=(4/7)x=1200
So, Amy originial has $2100 and Bob has $1200
(b)
The sum of money
=x+y
=2100+1200
=$ 3300

Maybe you could use:
Ley Y be the sum of money,
then (7-5)/11Y=$600
we get Y=$3300