I think whoever set the question has made the same mistake as Low Level Pilot.
While the Dry Adiabatic Lapse Rate is approximately 3.0C / 1000' you also need to consider the Dew Point Lapse Rate - which is around 0.5C / 1000'.
Combine these and you get 2.5C / 1000' - hence 400' for every degree C difference between Ambient and Dew Point on the ground.
OC619
P.S. That said the aim of the OP is to pass the exam - so go with the wrong approach while sitting the exam
Edited to add:
From this:
Dewpoint Lapse Rate
"we see that dew point and temperature converge at a rate of about 0.8K per 100"
Since 100m = 328'
an 0.8K = 0.8C
This translates to (around) 2.5C / 1000'