Math, low skilled immigrants often lacks basic education and can't properly read numbers. That makes them unfit even for most jobs we nowaday call "unskilled" like shop clerk etc. There are billions of such people in the world and when they immigrate there isn't much you can do to get them a job.

There is a reason why most unskilled jobs today require high school, because without basic math you can't do much at all. Immigrants with high school equivalent education or higher can integrate just fine, immigrants with lower level than that are hard to deal with so most countries refuse to take them in but those are the majority of people in low income countries.

In Sweden they are trying to teach these immigrants math etc, but it is hard since they both needs to learn a new language and math and all those standard things as an adult. And after that they have to compete with "unskilled" locals who learned those things as kids.