Each bicycle has 2 wheels and 1 seat, and each tandem bike has 2 wheels and 2 seats. So, we can write this mathmatically by writing one equation for the amount of seats and one for the amount of wheels.
If each bicycle has 2 wheels and each tandem bike also have 2 wheels, then the total number of wheels, 120, will be:
[tex]\begin{gathered} 2B+2N=120 \\ 2(B+N)=120 \\ B+N=60 \end{gathered}[/tex]Smilarlly, if each bicycle has 1 seat and each tandem bike has 2 seats, then the total number of seats, 68, will be:
[tex]\begin{gathered} 1B+2N=68 \\ B+2N=68 \end{gathered}[/tex]So, in the end, we have a system of two equations:
[tex]\begin{gathered} B+N=60 \\ B+2N=68 \end{gathered}[/tex]