Question 1201373
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Hi. How can I solve this? Thanks.
Both Mr Gan and Mr Koon received a salary every month. Last month, Mr Gan's salary was 9/10 of Mr Koon's salary. This month, Mr Gan's salary decreased by $220 while Mr Koon's salary increased by $200. In the end, Mr Gan's salary was 7/10 of Mr Koon's salary. What was Mr Gan's salary last month?

Let Gan's salary, last month, be G
Since last month, Mr Gan's salary was {{{9/10}}} of Mr Koon's salary, Mr. Koon's salary, last month, was {{{matrix(1,3, (10/9)G, "=", 10G/9)}}}
With Gan's salary decreasing by $220, Gan's salary, this month, is G - 220.
With Koon's salary increasing by $200, Koon's salary, this month = {{{10G/9 + 200}}}

As Gan's salary, this month, was {{{7/10}}} of Koon's, this month, we get: {{{matrix(5,3, G - 220, "=", (7/10)(10G/9 + 200), G - 220, "=", (7/10)(10/1)(G/9 + 20), G - 220, "=", (7/cross(10))(cross(10)/1)(G/9 + 20), G - 220, "=", 7(G/9 + 20), G - 220, "=", 7G/9 + 140)}}} 
                                                                 9(G - 220) = 7G + 140(9) ------ Cross-multiplying
                                                                9G - 220(9) = 7G + 140(9)
                                                                    9G - 7G = 140(9) + 220(9)
                                                                         2G = (140 + 220)(9)
                                                                         2G = 360(9)
                                 <font color = red><font size = 4><b>Mr. Gan's salary, last month</font></font></b>, or {{{highlight_green(matrix(1,7, G, "=", 360(9)/2, "=", 180(9), "=", highlight("$1,620")))}}}</pre>