3.1. How many numbers from 1 to 1000 can be represented as $[2 x]+[4 x]+[6 x]+[8 x]$ with positive $x$?
3.1. Let $2 x=y$. Note that the expression $[y]+[2 y]+[3 y]+[4 y]$ when $y$ increases from $n$ to $n+1$ takes values from $10 n$ to $10 n+6$, except for $10 n+3$, so all numbers ending in the digits $0,1,2,4$, 5,6 are representable in the given form. There are exactly 600 such numbers from 1 to 1000.