"whose tasks" is correct and usable in written English.
You can use it when referring to something that belongs to someone so as to ask for clarification or to make a comparison. Example sentence: Compare the lunches of students whose tasks require more physical activity.
These are people whose tasks include clearing, verifying and educating; their work is beyond essential.
Mr. Jaber became a member of the community advisory board, one of whose tasks was to assess potential survey questions.
MOBILIZING THE COALITION -- The Bush administration is working with fluid coalitions of friendly countries whose tasks are still undefined.
It was the first criminal investigation of an Israeli president, whose tasks are largely ceremonial but who is the official head of state.
Dr. Albright targeted the Bureau of Intelligence and Research for punishment, transferring a top administrator and an analyst and placing its overall security under the Bureau of Diplomatic Security, whose tasks include protecting American embassies and envoys.
Working with fluid coalitions of friendly countries whose tasks are still undefined, President Bush warned the Taliban government of Afghanistan today that "full warning has been given, and time is running out".
Each group had its own leader, whose task it was to defend its views.
The uniformed security guards are specially hired job-seekers, whose task it is to sit around doing nothing at all.
The emotional engagement with climate change is particularly relevant for those whose task is to make their companies more "sustainable".
Also revealed is the existence of Task Force 373 – a covert operations unit whose task is to "remove" the enemy.
Anyone whose task was to monitor events in the CAR knew the atrocities were close to inevitable.
