After charging for 1/3 of an hour, a phone is at 2/5 of its full power. How long will it take the phone to charge completely

If 1/3 of an hour results in 2/5 of full power, double both. You are left with 2/3 of an hour resulting in 4/5% or 80% power. If 1/3 results in 2/5, divide both sides by two. The remaining 1/5 power will be gained in an additional 1/6 of an hour. 2/3 + 1/6 = 5/6 of an hour to gain full power