2420.31 – Watering Your Venus Flytrap

I have two venus flytrap plants, each of which I lovingly water every day. Each plant gets the same amount of water as the other one, each day, and the daily amount is always the same. Through experimentation I found that one liter of water would last 14 days for the first plant alone, but would only last 10 days on the second plant alone (it's larger). How long would it last if I used the liter for both plants?

Solution

If I use my liter on both plants, on the first day I will use $\frac{1}{10} + \frac{1}{14} = \frac{24}{140} = \frac{6}{35}$ of the liter. In five days, I will use $5 \frac{6}{35} = \frac{6}{7}$ of the liter. That leaves $\frac{1}{7}$ left, which is not enough for a sixth day of watering.