Francesca wants to start exercising to burn at least 1500 extra calories per? week, but she does not have much spare time for exercise. according to a? website, she can burn an average of 3.5 calories per minute? walking, 4 calories per minute? cycling, and 8 calories per minute swimming. she would like her total time walking and cycling to be at least 3 times as long as she spends swimming. she would also like to walk at least 20 minutes per week. how much time should she spend on each activity not only to meet her goals but also to minimize her total exercise time per? week? what is her minimum exercise time per? week?