Hello, thank you for taking the time to post your question!
First you want to find the amount of the increase by taking
55 – 17 = 38
Once you have that you can divide by the original total and multiply by 100 to convert it over into a percentage
38/17 x 100% = 223.53%
So the percent increase in the number of volunteers is about 223.53%
I hope that helps get you moving in the right direction! Feel free to reach out if you have any additional questions beyond that :)