Answer:
a. 0.5 grams b. 0.25
Explanation:
Just use the half-life formula(I linked it below)
a. 4(1/2)^90/30=1/2
b. 4(1/2)^120/30=1/4
d it Acts between charged particles
70 percent of Earth is covered by water meanwhile 30% is covered by land.
3