From the table, find the number of vacuums sold $3$ or $4$ months after manufacture:

$\text{3 months} = 301, \quad \text{4 months} = 297$

The total number of vacuums is $1,000$. The probability is:

$P(\text{3 or 4 months}) = \frac{301 + 297}{1,000} = \frac{598}{1,000} = 0.598$