[tex]v_1-boat\ speed\\v_2-current\ speed\\\\ \underline{+\left\{\begin{array}{ccc}v_1+v_2=15\\v_1-v_2=7\end{array}\right}\ \ \ \ |add\ both\ sides\\.\ \ \ \ 2v_1=22\ \ \ \ |divide\ both\ sides\ by\ 2\\.\ \ \ \ \ \boxed{v_1=11}\\\\subtitute\ the\ value\ of\ v_1\ to\ the\ first\ equation\\\\11+v_2=15\ \ \ \ |subtract\ 11\ from\ both\ sides\\\boxed{v_2=4}[/tex]
Answer: boat speed is 11 km/h and current speed is 4 km/h.
