Answer:
The number they would count is
![\approx 0.49\cdot10^(-2)\ \%}](https://img.qammunity.org/2020/formulas/mathematics/high-school/vujtqx2l89i3m726lmwvwq9lwz0tbe5yke.png)
Explanation:
One mole is equal to 6.02214076x10^23 particles. In order to know the number counted by every human on Earth we simply multiply the number of humans times the count a "good counter" can achieve, 4x10^9x7.44x10^9 (for USA, change to 4x10^12 if you use de British English definition). Thus:
![\displaystyle{(4\cdot10^9\cdot7.44\cdot10^9)/(6.02214076\cdot10^(23))=(2.976\cdot 10^(19))/(6.02214076\cdot10^(23))\approx 0.49 \cdot 10^(-4)\approx 0.49\cdot10^(-2)\ \%}](https://img.qammunity.org/2020/formulas/mathematics/high-school/jm9yztws3jzr1byv141cfady1jp76cz58r.png)