To find David's yearly salary at his old job, we need to first determine how much he made in a month at his old job.
Let's call David's monthly salary at his old job "x".
According to the problem, David's new part-time job pays $2400 per month, which is 5 times what he made at his previous job.
Therefore, we can set up the equation:
5x = $2400
To solve for x, we divide both sides of the equation by 5:
x = $2400 / 5
Simplifying:
x = $480
So, David made $480 per month at his old job.
To find his yearly salary at his old job, we multiply the monthly salary by the number of months in a year (12):
Yearly salary = $480 * 12
Calculating:
Yearly salary = $5760
Therefore, David's yearly salary at his old job was $5760.