Times may be tough for many Americans, but, as the quality of public education grows shakier, it still pays to be a tutor — or at least a corporate tutoring firm. While the rest of the economy has sputtered and stagnated, the "supplemental education" sector has grown tenfold over the last decade. Here, a brief guide, by the numbers to "America's tutor boom":

$5 billion
Estimated size of the "supplemental education" sector

$500 million
Estimated size of the "supplemental education" sector in 2001, according to Michael Sandler, the founder of education consulting firm Eduventures

More than 1,400
Number of Kumon Math and Reading Centers across the country, making it the nation's largest tutoring chain. That's "nearly as many as familiar chains like Popeye's and Denny's," says Missy Sullivan at SmartMoney.

Number of Kumon students tutored annually

$95 to $150
Cost for one month of Kumon tutoring in a single subject

Number of drill sheets that Kumon students must master to complete the math program. The sheets start with basic arithmetic and end with basic calculus. Learning to ace all of them can take five years, according to Joe Nativo, Kumon's chief financial officer. Some have criticized Kumon's methods, dubbing them "mind-numbing" and "kill 'em and drill 'em."

Percentage of Kumon students who complete the entire math program, according to Nativo

3 to 5
Age range for the Junior Kumon program. "There's a table in the back of the Kumon center where preschool aged kids can learn vocabulary, reading and math, seemingly getting a leg up on their future fellow competitors, er, classmates," says Liz Moyer in The Wall Street Journal. "Scared? Me, too."

More than 50
Number of franchised tutoring firms in the country. "The tutoring field is increasingly centralized... and corporate," says Sullivan.

$45 to $60
Average cost, per hour, of tutoring at the major chain firms. Tutoring is typically not private; rather a tutor usually works with three students at once, coming around to each student intermittently in a "round-robin style."

More than 50
Amount, in percent, that the tutoring industry has grown since 2008, despite the economic downturn, according to Eduventures. "We're somewhat recession-resistant," says Kumon's Nativo.

Cost, per hour, for one-on-one test prep from one of the Princeton Review's "premier" tutors, the company's most elite and expensive academic helpers. A mere "private" tutor can be had for just $115 an hour. "Perhaps outside-of-school tutoring wouldn't be a $5 billion industry if parents could relax and trust their schools to do the educating," says Moyer. "But who does that nowadays?"

Number of states that have cut spending for K-12 education since 2008, according to the Center on Budget and Policy Priorities

Sources: Center on Budget and Policy Priorities, Council for American Private Education, Kumon, SmartMoney, Wall Street Journal