Determining the Equivalence Ratio
$$\begin{align}
\text{AFR (Air to Fuel ratio)} &amp;= \frac{n_{\text{air}}}{n_{\text{fuel}}} \\
\text{Equivalence ratio} &amp;= \frac{\text{AFR}}{\text{AFR}_{\text{stoich}}} \\
\end{align}$$

According to the problem statement, if you did everything right,
$$\begin{align}
\text{AFR} &amp;= 37.54 \\
\text{AFR}_{\text{stoich}} &amp;= \frac{\nu_{\text{oxygen}}}{\nu_{\text{fuel}}}\times 4.76 \\
\ce{C4H10 + $6.5$\,O2 &amp;-&gt; 4 CO2 + 5 H2O} \\
\text{AFR}_{\text{stoich}} &amp;= 4.76 \times 6.5 = 30.94 \\
\text{Equivalence ratio} &amp;= \frac {37.54}{30.94}=1.21
\end{align}$$
Now there are two types of equivalence ratio. One is air to fuel and another is fuel to air. I suspect in the answer they meant fuel to air equivalence ratio: 
$$\text{FAR equivalence ratio} = \frac{1}{\text{AFR equivalence ratio}} = \frac{1}{1.21}=0.82$$