A regression equation for predicting the average number of hours spent exercising per week by adults ages $50$ to $59$ ($y$) from the average for ages $40$ to $49$ ($x$) is $\hat{y}=2.5+0.65x$. The sample size is $n=10$, $\bar{x}=3.7$, $S_{xx}=5.4$, and $s=0.5$. Construct a $90\%$ prediction interval for $y$ when $x=4.2$.