To find the number of atoms in a cubic foot of sand, we can multiply the number of grains of sand in a cubic foot by the number of atoms in a single grain of sand.
- Number of grains in a cubic foot of sand: \( 4 \times 10^9 \) grains
- Number of atoms in a grain of sand: \( 6 \times 10^{19} \) atoms
Now, we calculate the total number of atoms in a cubic foot of sand:
\[ \text{Total atoms} = (\text{Number of grains}) \times (\text{Number of atoms per grain}) \] \[ = (4 \times 10^9) \times (6 \times 10^{19}) \]
Now we can multiply these values together:
\[ = 4 \times 6 \times 10^9 \times 10^{19} \] \[ = 24 \times 10^{28} \]
Next, we express \( 24 \times 10^{28} \) in proper scientific notation:
\[ = 2.4 \times 10^{29} \]
Hence, the expected number of atoms in a cubic foot of sand is \( 2.4 \times 10^{29} \).
The correct answer is A. \( 2.4 \times 10^{29} \).