A 150-pound person burns 6.3 calories per minute when walking at a speed of 4 miles per hour. While walking, this person eats a snack that has 90 calories. This snack subtracts from the calories burned while walking. How long must the person walk at this speed to burn at least 200 calories ?

the person should walk at least 3.038≈3 miles to burn 200 calories

Step-by-step explanation:

we have 2 contributions

a) reduction of calories by walking

burned calories by walking = calories burning rate * time, but time = distance travelled / velocity, then

burned calories by walking = calories burning rate * distance / velocity

b) increase in calories : snack calories = 90 calories

then

net burned calories = burned calories by walking - increase in calories

net burned calories = calories burning rate * distance / velocity - snack calories

replacing values

net burned calories = 200 cal, calories burning rate = 6.3 cal/minute, velocity = 4 miles / hour * 1 hour/60 min = 0.066 miles/minute, snack calories=90 cal, distance = X

then

200 cal = 6.3 cal/min / (0.066 miles/minute) * X - 90 cal

X = (200 cal + 90 cal) * 0.066 miles/minute/6.3 cal/min = 3.038 miles