[tex]total \: initial \: momentum \: = total \: final \: momentum \\ 0 \: = \: pboy \: + pgirl \\ 0 \: = \: (mv)boy \: + \: (mv)girl \\ 0 \: = \: (70.0 \: kg)(0.75 \: m|s) \: + \: (m)( - 0.90 \: m|s) \\ 0 \: = \: 52.5 \: kg. \: m|s \: + \: (m)( - 0.90) \: m|s) \\ - 52.5 \: kg. \: m|s \: = \: (m) \:( - 0.90 \: m|s) \\ \frac{ - 52.5 \: kg.m|s}{ - 0.90 \: m|s} \: = \: \frac{(m)( - 0.90 \: m|s}{ - 0.90 \: m|s} \\ 58.3 \: kg \: = \: m \\ \\ the \: mass \: of \: the \: girl \: is \: 58.3 \: kg.[/tex]