The Indo-European root ar- has the sense "fit together" and gives rise to art (with artisan, etc.), articulate, arthro- (joint, with arthritis, arthropod, etc.), arm, and harmony (from Greek 'joint' or 'shoulder'). The slight variant Indo-European root ra gives rise to rate, ratio, reason, read, and riddle.
Arithmetic, in part of its history, was arismetryk, whose components mean 'the art (ars) of counting (metric)'.
Related mathematical terms
Rate, Ratio, Rational, Irrational,