(b) A train of length 120 m is travelling at 144km/h. It passes under a bridge of width 20m.
Find the time taken for the whole train to pass under the bridge.
Give your answer in seconds.

Aclassmate thinks that solving a system by graphing gives an exact answer when the lines appear to cross at a grid point, but only an approximate answer when they don't. explain why this isn't true.