Question 375944
the mass is decreasing exponentially, giving half the mass at half-life tl : 

m(t)=m0*exp(-at/tl), m(tl)=m0exp(-a)=m0/2 => a=ln(2)
 
m(t)=m0*exp(-ln(2)t/tl)

here t=4000, tl=5715, m0=21g, 
 
m(4000)=21*exp(-ln(2)4000/5715)=21*0.6156=12.93g
 
 

 
let m=m0*exp(-ln(2)t/tl), find m0 : 
 
m0=m*exp(ln(2)t/tl)