For tri is substituted trayas when another numeral follows but not in a bahuvrīhi samāsa or before āśītiḥ,
As, त्रयोदश, त्रयोविंशतिः, त्रयस्त्रिंशत् ॥ But not in त्रैमातुरः (the second word not being a Numeral), nor in त्रिदशाः which is a Bahuvrihi, 2.2.25 त्र्यशीति ॥ This substitution takes place upto hundred : not here, त्रिशतम्, त्रिसहस्रम् ॥,