I have two large plants to water. A liter of water lasts 14 days if I use it on the first plant. It lasts 10 days if I use it on the second plant. How long would it last if I used it for both plants?
If I use my liter to water both plants, on the first day I will use 
of the liter.
In 5 days I’ll use 
of the liter, with 
left — not quite enough for the sixth day.