To find David's yearly salary at his old job, we can start with his new salary.
David's new job pays $2400 per month. Since this amount is 5 times what he made at his previous job, we can find his monthly salary at his old job by dividing his new salary by 5:
\[ \text{Old monthly salary} = \frac{\text{New monthly salary}}{5} = \frac{2400}{5} = 480 \]
Now, to find the yearly salary at his old job, we multiply the monthly salary by 12 (the number of months in a year):
\[ \text{Old yearly salary} = \text{Old monthly salary} \times 12 = 480 \times 12 = 5760 \]
Therefore, David’s yearly salary at his old job was $5760 a year.