The u. s postal service handles 170,000,000,000 pieces of mail each year. this is 40% of the worlds total. how many pieces of mail are sent each year

  40      170,000,000,000
  =
  100                x

Multiply diagonally to solve
So you get 40x = 17,000,000,000,000
Then divide each side by 40 to get x alone
So the answer is 42,500,000,000,000