中二數學問題---比

A有玻璃珠子的數目與B玻璃珠子的數目之比為5:2 ,
A有n顆 , 若A將她其中的18顆送給B , 兩人將會有相同的數目的玻璃珠子,
求n。
設A有n顆, B有x粒
n : x = 5 : 2..........................(1)
n - 18 = x + 18......................(2)
From (1),
2n = 5x
n = (5x)/2.............................(3)
Substitute (3) into (2)
(5x)/2 - 18 = x + 18
5x / 2 = x + 36
5x = 2x + 72
3x = 72
x = 24 //
Substitute x = 24 into (3)
n = 120/2
n = 60 //
n是60

n-18 = (n)÷( 5/5+2 ) x ( 2/5+2 ) + 18
n - 18 = (n)(2/5)+18
5n - 90 = 2n + 90
3n = 180
n = 60