Answer :

20 presidents are George Washington,John Adams,Thomas Jefferson,James Madison,James Monroe,John Quincy Adams 
Andrew Jackson,Martin Van Buren,William Henry Harrison,John Tyler,James Knox ,PolkZachary TaylorMillard, FillmoreFranklin PierceJames BuchananAbraham LincolnAndrew JohnsonUlysses Simpson GrantRutherford Birchard HayesJames Abram Garfield