A Numerical Argument For Being Selfish

I’ve had this argument in my head for a while, and I’d like to get it written down and hear what people think about it. To be clear, this is a thought experiment and I do not necessarily conduct myself or live my life in any way because of this argument. However, on the occasion that I do feel like I’m being selfish with my time maybe it provides me with some justification. Here’s the argument: Let’s say that there is a noble cause that I believe in. Let’s use solving homelessness as an example because it is massive and undeniably benevolent. Let’s say that there are 100,000 (arbitrary large number) people working on solving homelessness. If I contribute my efforts to the cause my contribution is 1/100,000 of the total effort assuming equal effort for each person. So my effort is 0.001% of the total effort. Now let’s say I focus my efforts on something selfish like my career. I am probably the only person working for the cause, so I am 1/1 of the total effort going towards it, or 100% of the total effort. Therefore, based on the percent of my effort relative to total effort it is more efficient for me to spend my time focusing on something selfish like my career. In other words, my effort will be more impactful for furthering my career than it would be for solving something like homelessness. Like I hinted to at the beginning, I don’t think that this argument is something that anyone should base their actions off of unless you want to end up old and unhappy. However, I do think that it is an interesting argument that might have something to it.