Short addition:
To be more exact:
{Q11=(if(Q2=="b","a",22))}
See here:
https://www.limesurvey.org/forum/can-i-do-this-with-limesurvey/112144-pre-fill-or-set-default-answer-to-list-radio-question-based-on-previous-answer
Joffm
If question "2" is 'b' question "11" will be locked with "a"
To be more exact:
{Q11=(if(Q2=="b","a",22))}
See here:
https://www.limesurvey.org/forum/can-i-do-this-with-limesurvey/112144-pre-fill-or-set-default-answer-to-list-radio-question-based-on-previous-answer
Joffm