Want to return name of last defined variable without explicitly naming it

I think there must be a better way to accomplish the OP's ultimate goal than the general approach outlined above. But be that as it may, the following culls all the Set[] commands from the last input line (so it has a little broader scope than what was asked for).

lastInputSets[] := Block[{Set},
   Grid@Cases[Hold[In[#]] &[$Line - 1] /. DownValues[In], 
     Set[lv_, v_] :> {HoldForm[lv], ": ", lv}, Infinity]
   ];

Examples:

foo = 10; murf = 20 + 30;
lastInputSets[]

The OP's example:

dec = 7.1 + 6.6 + 27.2 + 66.5;
lastInputSets[]
jan = 7.1 + 6.6 + 27.2 + 66.5 + 65.7;
lastInputSets[]
feb = 7.1 + 6.6 + 27.2 + 66.5 + 65.7 + 69.7;
lastInputSets[]

Or all at once:

(dec = 7.1 + 6.6 + 27.2 + 66.5;
 jan = 7.1 + 6.6 + 27.2 + 66.5 + 65.7;
 feb = 7.1 + 6.6 + 27.2 + 66.5 + 65.7 + 69.7;)
lastInputSets[]

If these symbols are newly defined, you could do

$NewSymbol = (lastsymbol = #) &;

dec = 7.1 + 6.6 + 27.2 + 66.5;
Print[lastsymbol, ": ", %]
jan = 7.1 + 6.6 + 27.2 + 66.5 + 65.7;
Print[lastsymbol, ": ", %]
feb = 7.1 + 6.6 + 27.2 + 66.5 + 65.7 + 69.7;
Print[lastsymbol, ": ", %]

dec: 107.4

jan: 173.1

feb: 242.8

---

In response to the comment -- as a more general purpose tool, one could use $PrePrint to automatically print the input and output from each evaluation. This uses Downvalues[In] as in @MichaelE2's answer:

$PrePrint = (
  Print[TraditionalForm@HoldForm[In[line] = #] /. line -> $Line /. DownValues[In]]; #
) &;

Now you have, for example:

Integrate[x^2, x]

$\int x^2 \, dx=\frac{x^3}{3}$

x^3/3

a = 2

$(a=2)=2$

2

Or you could just set the output to appear this way, without printing on a separate line, using

$PrePrint = (TraditionalForm@HoldForm[In[line] = #] /. line -> $Line /. DownValues[In]) &;

Use $PrePrint =. to turn this off.

I agree with MichaelE2, though, that depending on why you want to do this, there's probably a better way to go about it.