You got the correct answer, but you could clean up your reasoning as follows:
Define A to be total allowance, F to be allowance spent on food, and S to be allowance spent on stationary.
Since she spent 35 more on stationary than on food, then $35+S=F$. Also, $F=(2/3)A$ and $S=(5/6)(A-F)=(5/6)(A-(2/3)A)=(5/6)(1/3)A=(5/18)A$
Using substitution, $35+(5/18)A=(2/3)A$ so $A=90$.