Formula Error - The value entered does not match the format

I am trying to make a calculation within my form using the following JavaScript formula:

var a=this.getField("Number of meetings during period");
var b=this.getField("Number of meetings attended during period");
event.value=(b.value/a.value)

However, I am getting the following error message: The value entered does not match the format of the field {Detail Attendance Percentage}.

I have var a and var b formats set as a number and the field where the percentage is calculated formatted as a percentage.

I do not have a vast knowledge of JavaScript and was hoping someone could assist. I have read previous posts with regard to the error message however I have not been successful with the suggested solutions.

Thank you in advance for any assistance/guidance.

JC