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Gambling drama '21' stayed on top of the box office for a second weekend, making another $15.1 million, according to studio estimates.

The film follows a group of MIT brainiacs who join a card-counting scheme and make millions at Las Vegas casinos and co-stars KEVIN SPACEY, LAURENCE FISHBURNE, AARON YOO and LIZA LAPIRA.

In second and third were two premieres: GEORGE CLOONEY's screwball comedy 'Leatherheads' was No. 2 with $13.5 million, while 'Nim's Island' was just behind, with $13.3 million.

'Leatherheads' stars George as a charming 1920s football hero determined to guide his ragtag team to victory, all the while vying against rookie teammate JOHN KRASINSKI for a spitfire news reporter's (RENEE ZELLWEGER) affections.

'Nim's Island' finds JODIE FOSTER as an adventure writer who's never ventured out of her house -- until she has to save her No. 1 fan, played by ABIGAIL BRESLIN. GERARD BUTLER portrays the fictional hero she created who helps spur her on in her real-world rescue.

'Horton Hears a Who!' stayed strong in fourth with $9.1 million. JIM CARREY, in his second DR. SEUSS film (he also starred in 'How the Grinch Stole Christmas') plays an elephant who learns that a planet containing a village called "Who-ville" is located on an infinitesimal grain of dust and it's up to him to protect the people who live there. STEVE CARELL plays the Mayor of Who-ville in the animated film, while DANE COOK, SETH ROGEN, CAROL BURNETT, JAIME PRESSLY, ISLA FISHER, JONAH HILL ('Superbad'), WILL ARNETT ("Arrested Development") and AMY POEHLER ("Saturday Night Live") also provide voices.

'The Ruins' premiered in fifth with $7.8 million. The horror film stars JENA MALONE ('Into the Wild,' 'Saved!') in the story of a group of friends whose vacation to Mexico goes frighteningly sour after a visit to an archaeological site.