How to balance acid-base equations #2

Balance this acid-base equation: $H_3AsO_4+P_2O_5\to H_3PO_4+As_2O_5$.

SOLUTION:
Start writing the chemical equation with letters indicating the stoichiometric coefficients:
${\color{red}a }H_3AsO_4+{\color{red}b }P_2O_5\to {\color{red}c }H_3PO_4+{\color{red}d }As_2O_5$
Now balance every atom species for the principle of conservation of mass:
$\begin{cases}As:a=2d\\ P:2b=c\\ O:4a+5b=4c+5d\\ H:3a=3c\end{cases}$

Try now to express every variable in terms of $a$. From the arsenic equation, we know that $d=\frac{1}{2}a$; from the hydrogen equation we know that $c=a$.
From the phosphorus equation we have: $2b=c=a\Rightarrow 2b=a\Rightarrow b=\frac{1}{2}a$. To verify, we can try to substitute the value of $b$, $c$ and $d$ (in terms of $a$) in the oxygen equation (if correct, we will obtain $a=a$).
$\begin{cases}a=a\\ b=\frac{1}{2}a\\ c=a\\ d=\frac{1}{2}a\end{cases}$

Now give to the stoichiometric coefficient $a$ the value $2$. So we have:
$\begin{cases}a=2\\ b=1\\ c=2\\ d=1\end{cases}$

The balanced chemical equation is:
${\color{red}2 }H_3AsO_4+{\color{red}1 }P_2O_5\to {\color{red}2 }H_3PO_4+{\color{red}1 }As_2O_5$
That is:
${\color{red}2 }H_3AsO_4+P_2O_5\to {\color{red}2 }H_3PO_4+As_2O_5$

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PROBLEM 8:
Try to balance these two chemical equation:
$H_2SO_4+B(OH)_3 \rightarrow B_2(SO_4)_3+H_2O$
$ZnSO_4+Li_2CO_3 \rightarrow ZnCO_3+Li_2SO_4$