A 30-year-old woman achieves her target heart rate on a cool day by jogging at 6 mph. If she were to exercise on a very hot day, she would have to .

decrease the speed of her run to achieve her target heart rate.

Explanation:

In the hot day, the body will radiate out heat through the blood flowing in the veins and the arteries. The cardiovascular system works hard to radiate out heat from the body and provide cooling through perspiration as the surrounding environment is hot and heart needs to work harder to maintain the inner body temperature. Also, due to the perspiration, the electrolytes loose from the body thus the rate of heart increases to pump more blood so as to regulate the change in the electrolytic balance. In winters the surrounding environment of the body remains cool and the body perspire less than in summers.

Thus based on above description, the woman if reduce her speed of run then her target heart rate may achieve.