Remember that the general decay equation is:

where

is the amount after a time


is the initial amount

is the the decay percent in decimal form
The first ting we are going to do is find

by dividing our <span>decay rate of 25% by 100%: </span>

.
We also know from our problem that

. Lets replace

and

in our formula:


We know now that our decay rate is 0.75, and since 0.75<1, we can conclude that
this situation represents exponential decay.
Now, to find the initial amount, we are going to solve our equation for

:


Notice that

will depend on the number of ours

. <span />
the answer is 5 bebe. remember to use pemdas
p-parenthesis
e-exponents
m-multiplication
d-division
a-addition
s-subtraction
There will be eight marbles on the 9th line
1 centimeter is equivalent to 10 millimeters. So 15x10=150 millimeters.
Hope this helped! :D