For a previously evaluated cell in a Jupyter notebook, you can use `Out[...]`

,
replacing `...`

by the number of the output cell you want to refer to.

Example.

```
In[1]: 1 + 1
Out[1]: 2
In[2]: 20 + 20
Out[2]: 40
In[3]: Out[1] * 5
Out[3]: 10
```

EDIT (2018-01-05).

Note that this also works in the Sage REPL (read-eval-print loop), or "command-line interface",
ie, using Sage in a terminal. The reason is that, like Jupyter, it uses IPython behind the scenes.
The difference is that the cell numbers are not displayed there... They still exist though.

Example:

```
$ sage -v
SageMath version 8.1, Release Date: 2017-12-07
$ sage -q
sage: 1 + 1
2
sage: 20 + 20
40
sage: Out[1] * 5
10
```

If you want to get one of the last few cells and want to figure out what is the current number,
you could ask the length of `In`

or the length of `Out`

.

Example:

```
$ sage -v
SageMath version 8.1, Release Date: 2017-12-07
$ sage -q
sage: 1 + 1
2
sage: 9 * 9
81
sage: len(Out)
2
sage: Out[2]
81
```

There might be a way of making Sage display the numbering of the input and output...
But I don't know how to do it!