English to Italian Other Mathematics & Statistics

con Double si intende raddoppiare, ok. Ma "double to 1/5"?
Mi sto occupando di un libretto di enigmi e rompicapo matematici dell'antico Egitto.

Ahmes presents a number of problems in the Rhind Papyrus that are clearly meant to give instruction and practice with the issue of doubling any given unit fraction. As 2/3 was the only allowed fraction that wasn't 1/#, and repeating a fraction within a single number wasn't allowed, this was a somewhat thorny issue. Ahmes presented a table of fractions and their doubles - 1/5, for example, doubled to 1/3 +1/15 - but we have modern techniques instead.
Problem 21 of the Rhind Papyrus asks the reader to complete 2/3 +1/15 to 1. Can you do it following the Egyptian rules?

quel "to" devo renderlo con "per" o "a"?