Question 824847
x /a  + x/3a= 4-x

Solve for x
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Multiply thru by 3a
3x + x = 12a - 3ax
4x = 12a - 3ax
4x + 3ax = 12a
x*(4 + 3a) = 12a
{{{x = 12a/(3a + 4)}}}