Keeping one checked gamma triangle shows moving length scaling with rest length. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Five rest metres measure four moving metres
Rest length 5 m divided by gamma 5/4 gives moving length 4 m.
5÷45=4
Ten rest metres measure eight moving metres
Rest length 10 m gives moving length 8 m under the same gamma.
10÷45=8
Fifteen rest metres measure twelve moving metres
The third row keeps the same speed triangle. Rest length 15 m gives moving length 12 m.