Thanks that is what I got but the solutions say it’s 20 minutesWell, the question says Sam is walking at 3km/h, so you know you need to use that number and not the 5km/h one.
The rate given is per kilogram of body mass, so you need to multiply it by Sam's entire body mass to find how much energy Sam is going to use in 30 minutes. so 5.53kJ*65 which equals 359.45kJ (this is for every 30 minutes still).
This needs to be compared to how much energy is in the cappuccino, so we need to calculate that. It's given in kilocalories, but we need it in kJ to find out how fast Sam would use it since that's what we have it as. So we'll use the conversion given: 1 kilocalorie = 4.184 kJ. So 73*4.184 which = 305.432 kJ. (Technically you could also go the other way and say how many kilocalories does Sam burn in 30 minutes, not any easier, just different).
Also, you can see that the cappucino contains less energy than the amount Sam uses in 30 minutes. Therefore, you already know the answer can't be C or D because they are both greater than 30 minutes.
Now, because the rate of energy usage by Sam is the same every time, we can put it into a ratio where we can say "If Sam uses 359.45 kJ in 30 minutes, he will use 305.432 kJ in x minutes.
x/305.432 = 30/359.45
Multiply the denominators across each other to get
359.45x = 305.432*30
359.45x = 9192.96
Solve for x
x = 25.49
Therefore the answer is B, as the answer was approximately 25 minutes. Hope this helps!