chemistry
posted by Anonymous on .
A particular radioactive substance has a halflife of 4 minutes. How long will it take for 250 g of this radioactive material to decay to less than 0.1g?
24 min
36 min
40 min
48 min
72 min
I got 32 min but that not one of the choices. Please help

How many half lives does it go through to get from 250 g to < 0.1g?
# half lives x 4 min = ?