Please explain how to solve The solution given in ncert is not clear - Physics - Electric Charges And Fields

Question

Please explain how to solve The solution given in ncert is not
clear

Debalina Deb · Accepted Answer

Solution
<img alt="" src=
"https://s3mn%20mnimgs%20com/img/shared/content_ck_images/ck_6289082803af0%20jpg"
style="width: 933px; height: 1023px;">

Please explain how to solve. The solution given in ncert is not clear.