The Compendium of Kāśyapa 8.6.74

8.6.74

tadyathā--madhurāmlalavaṇakaṭuḥ madhurāmlalavaṇatiktaḥ madhurāmlalavaṇakaṣāya iti | uttare trayaḥ kaṭukatiktakaṣāyāḥ pūrvairmadhurāmlalavaṇairekaikaśo yujyante tataścatuṣkā niṣpadyante | tadyathā--kaṭutiktakaṣāyamadhuraḥ kaṭutiktakaṣāyāmlaḥ kaṭutiktakaṣāyalavaṇa iti | ete
catuṣkāḥ pañcadaśa pūrvasūtroktā nava dvitīyasūtroktāstrayaḥ tṛtīyasūtroktāstrayaḥ ityete catuṣkāḥ pañcadaśa

namely: sweet-sour-salty-pungent, sweet-sour-salty-bitter, and sweet-sour-salty-astringent. The latter three—pungent, bitter, and astringent—are combined individually one by one with the former—sweet, sour, and salty; thence quadruple combinations are produced, namely: pungent-bitter-astringent-sweet, pungent-bitter-astringent-sour, and pungent-bitter-astringent-salty. These quadruple combinations are fifteen: nine stated in the first rule, three stated in the second rule, and three stated in the third rule; thus these quadruple combinations are fifteen.