One wonders (well, I wonder) whether the numeracy skills are merely correlating with just some general braininess that might help people get ahead. I'd like to see the questions to see how numerate or innumerate these people are. I mean if the couple can't answer what 2 + 2 is, one doesn't expect high salaries. Nevertheless, "In addition to studying numeracy skills, the study also examined the impact that other cognitive skills, including memory retrieval and intact mental status, may have on financial outcomes. Researchers found the other cognitive functions studied had far less influence on a household's wealth."
ETA: According to NPR, here are the three questions used in the study. Not quite gimmes, but certainly not super difficult.
(1) If the chance of getting a disease is 10 percent, how many people out of 1,000 would be expected to get the disease?
(2) If 5 people all have the winning numbers in the lottery and the prize is $2 million, how much will each of them get?
(3) Let’s say you have $200 in a savings account. The account earns 10 percent interest per year. How much would you have in the account at the end of two years?