642/1,100 is equivalent to approximately 58.4%, therefore you can find what 58.4% of 40,000 is. In this case, it would be 23,360 people.
You can also divide 40,000 by 1,100 to find the amount it is increased by, which is 36.4. Then, you can multiply 642 by 36.4 to get a more accurate answer of 23,368.8 people.