Question 1200677
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x = amount of water to evaporate
amount is in liters


72% of 180 = 0.72*180 = 129.6 = amount of pure salt. 
This amount stays the same no matter how much water evaporates (since salt isn't carried into the air by the water particles).


A = 129.6 = amount of salt left after evaporation
B = 180-x = amount of solution left after evaporation
A/B = concentration of salt after evaporation
A/B = (129.6)/(180-x) 
A/B = 0.80


(129.6)/(180-x) = 0.80
129.6 = (180-x)0.80
129.6 = 0.80(180-x)
129.6 = 0.80(180)+0.80(-x)
129.6 = 144-0.80x
129.6+0.80x = 144
0.80x = 144-129.6
0.80x = 14.4
x = (14.4)/(0.80)
x = <font color=red>18</font>


Check:
(129.6)/(180-x) = 0.80
(129.6)/(180-18) = 0.80
(129.6)/(162) = 0.80
0.80 = 0.80
The answer is confirmed.


Answer:
<font color=red>18 liters</font> of water must evaporate.
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