[chiluvskk]

a first-order reaction has a half-life of 20.0 minutes. Starting with
1.00 x10^20 molecules of reactant at time t=0 , how many molecules remain unreacted after 100.0 minutes?


i really have no clue where to start, please help me!

[typhoon2028]

Are you familiar with the half life equation?

N(t) = N_0 2^{-t/t_{1/2}}. \,

N(t) is concentration or amount at time = t
N(0) is initial concentration
t = time
t1/2 = half life