Pressure / gas laws

In July 2003 there was an attempt to fly a manned, spherical balloon to a height of about
forty kilometres. At this height the atmospheric pressure is only one thousandth of its
value at sea level and the balloon would have expanded to a diameter of 210 m. The
temperature at this height is –60 °C. The attempt failed because the thin skin of the
balloon split while it was being filled with helium at sea level.
(a) Make an estimate of the temperature at sea level, and hence obtain the volume of
helium the balloon would have contained at sea level if it had been filled successfully.

Feeling pretty stumped by this. Any help would be paid in swag .

$\frac{p_1 V_1}{T_1}=\frac{p_2 V_2}{T_2}$

in order to find V₁

Still aren't getting it

You know the initial and final pressures
You know the final volume (via diameter)
You guess the initial temperature and you know the final

What don't you get?

Where are you getting the initial temperature from?

No it doesn't say calculate it says estimate.

Whatever value you use will give you the volume of the balloon at the start at that temperature.

If there was more to the question that you haven't told us about then it would help if you posted it if this info was there to help you "estimate" the initial temperature. As it stands from what you have posted, this is how you do the question.
Have you tried the calculation using 20 deg C, which is about the average surface temperature on earth?