I can do the first bit but I'm a little stuck on the second bit.
V =(4/3)pi.r3
dV/dt = 4.pi.r2.dr/dt so
dr/dt = -10-4
r = r0 - 10-4t
where (4/3)pi.r03 = 10-6 m3
r0 = 3√(3*10-6/4pi) m
We want the time when r = 0:
t = r0/10-4
(the units of 10-4 aren't given!).
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Thank you Alan.
