Manasara 9.35

9.35

aṣṭābhirvardhite ṛkṣaṃ hṛtvā śeṣaṃ kṣapiṣyate
navabhirvardhayetpaṅktiḥ hṛtvā śeṣaṃ vyayaṃ bhavet

When multiplied by eight, dividing [by twenty-seven], the remainder gives the lunar mansion. / One should multiply by nine and divide by ten; the remainder is the expense.