A researcher is comparing the average recovery times for two medications. Test the claim that the mean recovery time for medication A is less than that for medication B at the $\alpha =0.05$ significance level. Assume independent random samples and equal population variances. Sample statistics: ${\bar{x}}_A=4.2$ days, $s_A=1.1$, $n_A=15$; ${\bar{x}}_B=5.0$ days, $s_B=0.9$, $n_B=12$.