NEW YORK/SAO PAULO - Walmart Inc (WMT.N) said on Monday that it has sold an 80 percent stake in its Brazilian operations to private equity firm Advent International, exiting an underperforming business in its third major international deal since April. The world’s biggest retailer has been looking to jumpstart its overseas business by retreating from lower-growth markets and investing in places like China and India. Brazil had for a decade been the focus of expansion but the unit stumbled in recent years as operational issues compounded the effects of a deep recession.