VTMO 2021 — Grade 5 — vtmo Question 6

Tien, Son, Dung start with equal money. Tien buys $3$ pencils and $4$ notebooks, retaining $7 USD$; Son $3$ pencils and $3$ notebooks, retaining $28 USD$; Dung $1$ pencil and $4$ notebooks, retaining $21 USD$. Find each initial amount.