First annex zeros on the right to make them all have as many decimals places as
the one with the most number of decimal places:

-34.2500
-34.2510
-34.2050
-34.2519

Then remember how to tell whether one signed number is HIGHER or
LOWER than another.  Ask yourself this question:  If they represented
degrees Fahrenheit, which would be the LOWER temperature and which would be a
HIGHER temperature?   A LOWER temperature is represented by a LOWER number
and a HIGHER temperature is represented by a HIGHER number. So think of
them as if they were

-34.2500°F
-34.2510°F
-34.2050°F
-34.2519°F

The lowest (coldest) temperature of the four would be -34.2519°F, then
-34.2510°F, then -34.2500°F, and the highest temperature of the four would 
be -34.2050°F,
so

-34.2519 < -34.251 < -34.25 < -34.205

Edwin