recall your d = rt, distance = rate * time.
b = speed rate of the boat.
c = speed rate of the current.
keeping in mind that, as the boat goes Upstream, against the current, it's speed is not "b", but is really " b - c ", because the current is subtracting speed from it.
likewise, when the boat is going Downstream, because is going with the current, is really going faster at " b + c ".
[tex]\bf \begin{array}{lccclll}
&\stackrel{km}{distance}&\stackrel{kmh}{rate}&\stackrel{hours}{time}\\
&------&------&------\\
Upstream&305&b-c&5\\
Downstream&651&b+c&7
\end{array}
\\\\\\
\begin{cases}
305=(b-c)(5)\implies 61=b-c\implies 61+c=\boxed{b}\\
651=(b+c)(7)\implies 93=b+c\\
----------------------\\
93=\boxed{61+c}+c
\end{cases}
\\\\\\
93=61+2c\implies 32=2c\implies \cfrac{32}{2}=c\implies 16=c[/tex]
what is the speed of the boat? well, 61 + c = b.