Answer:
Step-by-step explanation:
The total number of bin is 21
the mean of 21 bins is
[tex]=\frac{21}{2} \\\\=10.5\\\\\approx11[/tex]
The standard deviation is 3 bins.
We calculate , by using z formula
[tex]P(x<7)=P(z<\frac{x-\mu}{\sigma} \\\\=P(z<\frac{7-11}{3} )\\\\=P(z<-1.33)\\\\=0.0918 (z \ \ table)\\\\\therefore P(x<7)=0.0918[/tex]
Since after 1000 balls is drop, we will mutiply with 1000
0.0918 x 1000
= 91.8
≅ 92
Therefore, we have 92 balls