Production will be stopped if the difference in diameter is greater than 0.002 mm.
This difference can mean that the golf ball has a diameter more than 0.002 larger than 42.67 mm, or a diameter more than 0.002 smaller than 42.67. This is the reason for using an absolute value function.
Absolute value represents the distance a number is from 0. For this function, we would want only the values where the function is less than 0.002.
The time required to get a total amount, principal plus interest, of $2,500.00 from simple interest on a principal of $2,000.00 at an interest rate of 4% per year is 6.25 years (about 6 years 3 months).
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The two distances are the same (out and back), so set them equal.
That is done by having a (rate)(time) equal a (rate)(time).