Question 273469
Let x=no. of packages delivered by SQ
  x+3=no. of packages delivered by RE
2(x+3)=no. of packages delivered by SD

x+(x+3)+2(x+3)=21
x+x+3+2x+6=21
4x+9=21
4x=21-9
4x=12
x=3

Plug x back into each of the three companies as listed above and you will have your answer.