How to edit the task by parent user using option "Controlled by Parent"

Unable to edit he task -System thronging insufficient privilege error 

Issue

1. Parent user  creating the activity/task  under  account.

2. Assigned to another person having same role.
3. And then trying to edit the same task (click edit on task)
4. System throws insufficient privilege error

Solution

Set Deafult internal access for task as "Controlled by Parent" in the below sharing setting screen and now user can able to edit the task.

Sharing Settings

This page displays your organization's sharing settings. These settings specify the level of access your users have to each others' data.

*Manage sharing settings for:

Default Sharing Settings

Organization-Wide Defaults

Object	Default Internal Access	Default External Access	Grant Access Using Hierarchies
Lead	Private	Private
Account, Contract and Asset	Private	Private
Contact	Controlled by Parent	Controlled by Parent
Opportunity	Private	Private
Quote	Controlled by Parent	Controlled by Parent
Case	Private	Private
Campaign	Private	Private
User	Public Read Only	Private
Activity	Controlled by Parent	Controlled by Parent
Calendar	Hide Details and Add Events	Hide Details and Add Events
Price Book	Use	Use