According to the American Heart Association [0], CPR can double or triple the chances of survival:

"Each year nationwide, more than 350,000 people suffer a cardiac arrest outside of a hospital; only about 10 percent survive.

When the heart stops beating, a lack of oxygenated blood can cause death within minutes. If CPR is performed immediately, it can keep blood flowing to the brain and other vital organs, doubling or tripling a person’s chance of survival."

[0] https://www.heart.org/en/news/2018/08/22/cpr-training-at-sch...