Charak Samhita
पृथगम्लादियुक्तस्य योगः शेषैः पृथग्भवेत्। मधुरस्य तथाऽम्लस्य लवणस्य कटोस्तथा ॥१६॥
Likewise, combination of three rasas together result in twenty substances. There are fifteen combinations of four rasas together. They are formed as follows: The group of sweet and sour tastes, combines with any two of remaining tastes beginning with salt in six different ways. They this form the ‘group of fours’ of tastes.
english translation
pRthagamlAdiyuktasya yogaH zeSaiH pRthagbhavet। madhurasya tathA'mlasya lavaNasya kaTostathA ॥16॥
hk transliteration by Sanscript