A toy mass and speed limit make rest energy exact. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The mass is a training number
This book uses toy mass 4 kg and toy speed limit 5 m/s; real particle masses use different units.
m=4c=5
Rest energy is mass times speed limit squared
The rest energy is 4 times 5 times 5, which is 100 J.