Three candidates Alice, Bob, and Calvin are up for election. The number of votes for Bob is greater than or equal to 50% of the votes for Calvin, and is less than or equal to 20% of the votes for Alice. The sum of the votes for both Bob and Calvin is greater than or equal to 251. What is the minimum number of votes for Alice?
b >= .5c
b <= .2a
b+c >= 251
a >= 1255/3, so a>=418