Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 2 · Signs, Planets, Birth
जातकम् १
दशागुणैर्हता भक्त्या गुणैक्येन समागताः ।शेषेऽर्कादिहते भक्ते मासाद्यैक्येन नारद ॥ १४१ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 55, verse 141.
Multiply the dasha (planet period) years by the multipliers. Divide by the sum of the multipliers: this gives years. Multiply the remainder by 12 and so on. Divide by the same sum: months and the rest, Narada.
Simple meaning: Periods are multiplied by their multipliers and divided to give years and months.
षड्वर्गार्द्धस्य त्रिंशस्य त्रिकोणगश्च स्मरगः ।
Planets share the sub-periods by their house from the period lord.
क्रमः केंद्रादिकोऽत्रापि द्वित्र्यादौ सबलस्य च ।
The period lord's share is 84, a planet with it 42.
त्रिकोणस्थस्य चाष्टाक्षिसूर्या द्यूनगतस्य च ।
Planets in the luck houses, the 7th, and the 4th or 8th get their own shares.