A patient has smoked for 10 years at one pack a day and 15 years at two packs a day. How many pack years does this equate to?

To calculate the total pack years for a patient who has smoked, the formula is to multiply the number of packs smoked per day by the number of years smoked. In this case, the patient smoked for two different periods with varying amounts.

For the first period, the patient smoked one pack a day for 10 years:

1 pack/day * 10 years = 10 pack years.

For the second period, the patient smoked two packs a day for 15 years:

2 packs/day * 15 years = 30 pack years.

Adding both periods together gives:

10 pack years + 30 pack years = 40 pack years.

This total of 40 pack years provides a measure of the cumulative exposure to smoking, which is important for assessing health risks related to smoking. Thus, the correct answer is based on the calculations of each smoking period contributing to the overall pack years accrued.