-2(2m-3) = -4m+6
2n-(4-2m) =2n-4+2m
16a-3(3b-4c) =16a-9b+12c
t-3分之2（24-9v) =t-16+6v
-(5m+n)-6(a-3b)=-5m-n-6a+18b
-2分之1(x+y)-4分之1(p-q)=-1/2x-1/2y-1/4p+1/4q