Straight-forward: given $x$ identical balls and $n$ boxes, how many different ways can we place the balls into the boxes? A box can be empty. example: x = 4, n = 3  |OOOO| - |    | - |    | |OOO | - |O   | - |    | |OOO | - |    | - | O  | |OO  | - |OO  | - |    | |OO  | - |O   | - |O   | |OO  | - |    | - |OO  | |O   | - |OOO | - |    | |O   | - |OO  | - |O   | |O   | - |O   | - |OO  | |O   | - |    | - |OOO | |    | - |OOOO| - |    | |    | - |OOO | - |O   | |    | - |OO  | - |OO  | |    | - |O   | - |OOO | |    | - |    | - |OOOO|  ANS = 15  In this case, seems like we have a sum $(1+2+3+4+5)$. example: x = 3, n = 4  |OOO| - |   | - |   | - |   | |OO | - |O  | - |   | - |   | |OO | - |   | - |O  | - |   | |OO | - |   | - |   | - |O  | |O  | - |OO | - |   | - |   | |O  | - |O  | - |O  | - |   | |O  | - |O  | - |   | - |O  | |O  | - |   | - |OO | - |   | |O  | - |   | - |O  | - |O  | |O  | - |   | - |   | - |OO | |   | - |OOO| - |   | - |   | |   | - |OO | - |O  | - |   | |   | - |OO | - |   | - |O  | |   | - |O  | - |OO | - |   | |   | - |O  | - |O  | - |O  | |   | - |O  | - |   | - |OO | |   | - |   | - |OOO| - |   | |   | - |   | - |OO | - |O  | |   | - |   | - |O  | - |OO | |   | - |   | - |   | - |OOO|  ANS = 20  In this case, seems like we have a sum $(1+3+6+10)$.
How to use the  method?  Say I want to find number of combinations I can get with $x_1+x_2+x_3+x_4=22$,  where $x_i\in\mathbb{N}$.  Is this the correct time to apply the method?  This is being repurposed in an effort to cut down on duplicates, see here:     and here:
I'm looking for one word which describes what a student who missed classes needs to do in order to complete his missing.   For example if he missed biology class he needs to seat with the teacher and study this missed class together with her and then he get the mark for this missed class. Someone -who isn't native English speaker- told me that it's called "rework". But I looked for it in some dictionaries (such as ) and I didn't find such definition.   : "SCHEDULE of reworks of missed classes" I install virtualBox on Ubuntu 16.04, but after install and create new virtual Host shows this error:     Kernel driver not installed (rc=-1908)      The VirtualBox Linux kernel driver (vboxdrv) is either not loaded or there is a permission problem with /dev/vboxdrv. Please install virtualbox-dkms package and load the kernel module by executing  modprobe vboxdrv       as root. If it is available in your distribution, you should install the     DKMS package first. 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