How much radiation will a person receive standing 100 feet from a source with a dose rate of 20 mR/hr where four exposures were made lasting three minutes each?

To calculate the total radiation dose received by a person standing at a certain distance from a radiation source, you start with the dose rate and determine how long the exposure lasts.

In this case, the dose rate is 20 mR/hr. If the person is exposed for three minutes at this rate, you first convert that duration into hours since the dose rate is given in terms of hours. Three minutes is equivalent to 0.05 hours (since 3 minutes ÷ 60 minutes/hour = 0.05 hours).

Now, you calculate the dose for one exposure by multiplying the dose rate (20 mR/hr) by the exposure time (0.05 hours):

20 mR/hr × 0.05 hours = 1 mR.

Since four separate exposures are noted, you multiply this dose by four:

1 mR/exposure × 4 exposures = 4 mR.

Thus, if each exposure contributes 1 mR, after four exposures the total dose received is 4 mR. This is why the total radiation dose received in this scenario is accurately articulated as 4 mR, correlating with the answer provided.