Packaging Corporation of America produces containerboard and white papers. WestRock Co$.$ manufactures paper products and corrugated products. Financial statement data for these two companies follow:

Packaging Corporation of America

$($$ in millions of dollars$)2015201620172018$

Sales $ $5,779\mathrm{.}0$ $ $6,444\mathrm{.}9$ $ $7,014\mathrm{.}6$

Accounts receivable at December $31$ $ $636\mathrm{.}5689\mathrm{.}2830\mathrm{.}7901\mathrm{.}9$

WestRock Co$.$

$($$ in millions of dollars$)2015201620172018$

Sales $ $14,171\mathrm{.}8$ $ $14,859\mathrm{.}7$ $ $16,285\mathrm{.}1$

Accounts receivable at December $31$ $ $1,575\mathrm{.}41,592\mathrm{.}21,886\mathrm{.}82,010\mathrm{.}7$

Source: Computed from amounts in the Packaging Corporation of America and WestRock Co$.$ Form $10-$Ks for $20142018.$ In $2015$ and $2016,$ WestRock was known as WRKCo., Inc.