I don't know who needs to hear this but year "n" is a leap year iff gcd(80, n) > gcd(50, n)—Lynn
@chordbug tweet 
hmm...
80 = 5 * 2*2*2*2
50 = 5*5 * 2
the additional "2" factors in 80 make a difference (by doubling the first gcd) every 4 years (2*2 and not 2, because there's one "2" in common)
—Phairupegiont @Phairupegiont tweet
the additional "5" factor in 50 makes a stronger (5>2) difference every 100 years (4*5*5 and not 4*5, because there's a "5" in common) but every 400 years the 2 factors reach 2*2*2, beating the 5..! slick!
it feels like cheating... 3 cases in just 2 magic numbers... ✨