(2/5) of (2/3) of students = 4/15 of students study French. Then 1-(4/15) = 11/15 of students do not study French. The ratio you want is
(4/15):(11/15) = 4:11
_____
In a math expression "of" means "times".
This is exponential growth/decay type problem...
F=Ir^t, F=final value, I=initial value, r=rate, t=time, in this case I=1250 and r=(1-.045)=0.955 so
F=1250(.955)^t and we want to find t for when F=800
800=1250(.955)^t
.64=.955^t take the natural log of both sides...
ln.64=t ln.955
t=(ln.64)/ln.995
t≈9.69 weeks
t≈9.69 weeks (to nearest hundredth of a week)
