The half-life of a radioisotope is 30 days. What will remain of a 20.0 g sample after 90 days?

After 30 days ( first half life) 120 /2 = 60 g decays and 60 g remains left.
After another 30 days ( two half lives or 60 days) 60 /2 = 30 g decays and 30 g remains left .
After another 30 days ( three half lives or 90 days) 30 /2 = 15 g decays and 15 g remains left.
After another 30 days ( four half lives or 120 days) 15 /2 = 7.5 g decays and 7.5 g remains left.
after four half lives or 120 days , 7.5 g of the isotope will be left.

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