Question 1210613
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I'd go with the last method tutor KMST mentioned.
<table border = "1" cellpadding = "5"><tr><td></td><td>Zaven</td><td>Marek</td><td>Total</td></tr><tr><td>Start</td><td>3x</td><td>900-3x</td><td>900</td></tr><tr><td>Spent</td><td>2x</td><td>1.5x</td><td>3.5x</td></tr><tr><td>Final</td><td>x</td><td>900-4.5x</td><td>900-3.5x</td></tr></table>


Refer to the given statement that <pre>"Marek had 75% of the total number of coins the two friends had left in the end"</pre> which means we'll use the last two entries of the bottom row to form  this equation
900-4.5x = 0.75*(900-3.5x)
That equation solves to x = 120


Plugging that value of x back into the table above produces the following:
<table border = "1" cellpadding = "5"><tr><td></td><td>Zaven</td><td>Marek</td><td>Total</td></tr><tr><td>Start</td><td>360</td><td>540</td><td>900</td></tr><tr><td>Spent</td><td>240</td><td>180</td><td>420</td></tr><tr><td>Final</td><td>120</td><td><font color=red>360</font></td><td>480</td></tr></table>


Answer: <font color=red>360 coins</font>
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