Expression manager is totally ready for this : www.limesurvey.org/manual/Expression_Man...#Access_to_Functions

It's mathematic/logic , not really LimeSurvey issue …

Maryam01 wrote: I would ask if can do willing to pay price change equation "W T P PRICE CHANGE" by lime survey.

This kind of questions are possible with LimeSurvey (Exception: a looping is not possible - definite amount of questions has to be defined), but in reality not often used. Most respondents don't like these kind of questions. I would never use them in a real experiment. Looks more like a student assignment, where students answer the questions
In reality even more random WTP methods fail, e.g. GaborGranger

en.wikipedia.org/wiki/Gabor%E2%80%93Granger_method


What version of LimeSurvey is used?
You will need to learn ExpressionScript ( www.limesurvey.org/manual/Expression_Man..._-_quick_start_guide ) and create a few Equations to create the new price.
LimeSurvey is not supporting loops, so you need to know how many questions are asked at maximum.
Should be possible, cause you won't find many people which answer more than five questions of that style in a row.