Suppose $100 dollars is deposited into an account and the account doubles every 8 years . how much will in the account after 40years ?

Yay, doubling time

the equation is


A=final amount
P=initial amount
t=time elapsed in years
d=doubling time in years

ok

A=A
P=100
d=8yrs
t=40yrs

so






there will be $3200 in the account after 40 years

Year 0   : D = $100
year 8   : D= $200
year 16 : D = $400
year 24 : D = $800
year 32 : D= $1600
year 40 : D= $3200

That's the longest way to do it :

40/8 = 5 ; the account is gonna doubles 5 times

25 is obviously the correct answer