sesquiduplicate

a. [ Sesqui- + duplicate. ] Twice and a half as great (as another thing); having the ratio of two and a half to one. [ 1913 Webster ]

Sesquiduplicate ratio (Math.), the ratio of two and a half to one, or one in which the greater term contains the lesser twice and a half, as that of 50 to 20. [ 1913 Webster ]