Not extendable

Every day John has a task of reading at least twenty pages, so at the end of the week he should finish one hundred forty pages, but even if he has finished thirty pages one day, it does not mean that the next day he can let up and read only ten pages; he must still read twenty pages the next day. What is this called?

-This task is not extendable.(=The extra work done cannot be used to set off the quota of tomorrow.)