Chef Ken used three special ingredients, $P$, $Q$ and $R$ to make a sauce. Each packet of Ingredient $P$ had a mass of $300\textasciitilde g$. Each packet of ingredients Q and R had a mass of $400\textasciitilde g$ and $500\textasciitilde g$ respectively. The ratio of the number of packets of ingredients $P$, $Q$ and $R$ used was $7 : 3 : 2$. Given that the total mass of the sauce was $172\textasciitilde kg$, how many packets of ingredients were used altogether? 
Overseas Competition->Knowledge Point->Calculation Modules->Ratios and Proportions