I think that there is a number missing here:

in the sense that

of the 24 members alreaday support the idea.

of 24 is
which means that 8 members already support the idea.
So if they need the support of 13 members, this means that they need 13-8=5, that is 5 more members.
This could help you
https://prezi.com/t7qmpi9mhz_p/roles-of-menwomen-and-children-of-the-mikmaq-tribe/
Answer:
Hand washing has external health benefits, helping prevent the spread of communicable diseases. If a program were somehow devised so that people received a small reward every time they washed their hands, we would expect the frequency of hand washing to increase.
Explanation: People who are given something (A reward of some sort) are proven to do something more, (in this case washing their hands more), or better then they would do without a source of motivation.