Let the laps run by the sister be x
The laps run by the boy is 2.5 times the laps by the sister. Thus the laps run by the boy is
[tex]2.5x[/tex]The total laps by the boy and the girl is 7. Therefore,
[tex]\begin{gathered} x+2.5x=7 \\ 3.5x=7 \\ x=\frac{7}{3.5}=2 \\ \text{The laps by sister is 2.} \\ 2.5x=2.5\ast2=5 \\ \text{Thus, the laps by boy is 5} \end{gathered}[/tex]