Chapter 15 - Electric Charge, Forces, and Fields - Learning Path Questions and Exercises - Exercises - Page 556: 8

a). Option (4) $\frac{1}{4}$ b). $9F$

$F=k\frac{q_{1}q_{2}}{r^{2}}$ a). When distance (r) is doubled, then F is decreased to $\frac{1}{4}$ of the original force. Hence (4) is correct. b). When distance (r) is reduced to one-third, then F is increased to $3^{2}=9$ times the original force i.e. $9F$