There are about 6×1019

To calculate the number of atoms in a cubic foot of sand, we can follow these steps:

1. Calculate the total number of atoms in sand by multiplying the number of grains of sand by the number of atoms per grain.

We know:
- Number of atoms in a grain of sand = \( 6 \times 10^{19} \)
- Number of grains in a cubic foot of sand = \( 4 \times 10^{9} \)

Therefore, the total number of atoms in a cubic foot of sand is:
\[
\text{Total atoms} = (4 \times 10^{9}) \times (6 \times 10^{19})
\]

2. Multiply these values:
\[
= 4 \times 6 \times 10^{9} \times 10^{19}
\]
\[
= 24 \times 10^{9 + 19}
\]
\[
= 24 \times 10^{28}
\]

3. Convert this to proper scientific notation, which would typically have the leading digit as a number between 1 and 10:
\[
= 2.4 \times 10^{29}
\]

So the number of atoms you would expect to find in a cubic foot of sand is \( 2.4 \times 10^{29} \).

The correct response is:
**2.4×10²⁹**.