Hi Anonymous Letter,
Here, we see 3 different demand curves - one intersecting at q=10, one at q=20 and one at q=40. The differences arise here because of the different prices of snacks in each of the scenarios:
1) Intersection at 10 => buying 0 juice Bobby can afford 10 units of snacks. As he has 20 euros to spend, it means the price per unit of snacks is 20/10=2. Then, the demand is tangent to the utility function where q=5 so we find one point of his demand curve to be p=2, q=5.
2) Intersection at 20 => p=20/20=1. Then tangent to the utility for q=7. So p=1, q=7 another point on the demand curve.
3) Intersection at 40 => p=20/40=0.5. Then tangent to the utility for q=10. So p=0.5, q=10 another point on the demand curve.
Thus, we see that answer b is one of the possible points we found.
Hope that helps!
Your Success Formula Team - Wojtek