Your Toothbrush Is Part Of The Problem

Before the introduction of the renewable heads , the percentage of the toothbrush waste among the total plastic waste after two years was approximately,

$\Large \frac{ 4.2 × 10^9 × (2.5 × 2_{\text{years}}) × (\frac{18}{1000}) }{ 345 × 10^9 × 2_{\text{years}} }$ $× 100$

$= 0.0547826\%$

After the end of two years, if everyone still uses $5$ toothbrushes in two years, but buys body only on the first toothbrush and then just change heads $5-1 = 4$ times, for the next two years, the percentage share of toothbrush plastic become,

$\Large \frac{ [4.2 × 10^9 × 2_{\text{years}} × (1) × (\frac{18}{1000})] + [4.2 × 10^9 × 2_{\text{years}}) × 4× \frac{1}{3} × (\frac{18}{1000})] }{ 345 × 10^9 × 2_{\text{years}} }$ $× 100$

$= 0.0255652\%$

Reduction in percentage volume of the waste,

$0.0547826 - 0.0255652 = \boxed{0.0292174\%}$

---

That's $0.0292174\%$ of $2 × 345 × 10^9$ kilograms. After the end of two years that's $2.016$ Billion Kilograms of biennial plastic waste reduced.