(d) Totally, a person covers 4.8 km. That means he covers 2.4 km on one side and 2.4 km on other side. So, distances he will be covering will be 20 + 40 + 60 + ... ∴2400=(
n
2
)(2×20+(n−1)20)=10n(n+1) After solving, we get n = 15 ∴ Total number of stones = 15 + 15 + 1 = 31