The velocity of boat m still water is 10 ms $m{s}^{-1}$ ${}^{}$ . The river flow velocity is 5 ms $m{s}^{-1}.$ ${}^{}$

If ${}_{}$ $v_b$ ,= $$ velocity of boat with respect to river velocity, $v_b=$ ${}_{}$ actual velocity of boat (i.e. with respect to bank), $v_r=$ ${}_{}$ velocity of stream, then the value of $v_b$ ${}_{}$ is