If you would like the achievement "Convince Jill", you will need to ask Jill to give you the achievement, by typing it into the chat box:
"Give me the achievement, please". She insists on the please, btw.

Then you will have to play a guessing game by guessing a number between 0 and 1000. Simply try and start with half increase or decrease. Use 500 as first guess, if it is higher 750, if it is lower 250 etc. This way you can guess the number real quick.