Lighting two candles so one is twice the other

Two identical candles burn at steady rates, one lasting $2$ hours and the other $1$ hour. You light both at the same moment and want the slower candle to be exactly twice as long as the faster one at 4:00 PM. When should you light them? Give the time on the 24-hour clock as a four-digit number (e.g. $2020$ for 8:20 PM).

1520. After $t$ hours the remaining fractions are $1-\tfrac t2$ and $1-t$. Solving $1-\tfrac t2=2(1-t)$ gives $t=\tfrac23$ hour, i.e. $40$ minutes, so light them at 3:20 PM.