Also see this: Change default precision - #47 by stevengj
using DecFP
using ChangePrecision
y = Dec128(5)/Dec128(36)-Dec128(7)*√Dec128(15)/Dec128(30)
@changeprecision Dec128 begin
z = 5/36 - 7*√15/30
end
y == z # true
EDIT: or more concisely:
@changeprecision Dec128 z = 5/36-7*√15/30