This week, 64-year-old reggae icon Jimmy Cliff releases Rebirth, an album produced by Rancid’s Tim Armstrong. It’s the latest in an ongoing trend that Flavorwire dubbed "inter-generational" musical collaborations: full-album pairings between an older legend and a younger industry stalwart. From Jack White and Loretta Lynn’s Van Lear Rose to Ben Folds and William Shatner's Has Been, these pairings have always been around. But the last few years have seen a marked uptick. And so now, in our never-ending quest to make everything a competition, we evaluate some of the more prominent inter-generational collaborations, not based on the merit of their music, but on the inherent difficulty level they possessed. Using our foolproof "Revival of Beloved But Forgotten Artist" Difficulty-O-Meter, we figure out exactly which young, elder-respecting musician/producer overcame the biggest challenge.