1. Use Raoult’s law for an ideal equimolar solution

For an equimolar mixture, $x_{\text{benzene}}=x_{\text{toluene}}=\frac{1}{2}$

Given vapour pressures at $20^\circ$C:

• Benzene: $P_{\text{benzene}}^0=70\ \text{torr}$
• Methyl benzene (toluene): $P_{\text{toluene}}^0=20\ \text{torr}$

So partial pressures are: $p_{\text{benzene}}=x_{\text{benzene}}P_{\text{benzene}}^0=\frac{1}{2}\times 70=35\ \text{torr}$ $p_{\text{toluene}}=x_{\text{toluene}}P_{\text{toluene}}^0=\frac{1}{2}\times 20=10\ \text{torr}$

2. Find total vapour pressure

$P_{\text{total}}=35+10=45\ \text{torr}$

3. Find mole fraction of benzene in vapour phase

$y_{\text{benzene}}=\frac{p_{\text{benzene}}}{P_{\text{total}}}=\frac{35}{45}=\frac{7}{9}=0.777\ldots$

4. Convert into the required form

Question asks for: $\underline{\hspace{2cm}}\times 10^{-2}$

Now, $0.777\ldots = 77.7\ldots \times 10^{-2}$

Nearest integer $=78$.

Therefore, the required answer is: $\boxed{78}$