Algebra

In order to solve this problem, we must find the common ratio of the numbers. 

Since it's a geometric series, setting r as the common ratio, we get

$r = .036 / .4 = 36 / 400 = 9 /100 = .09$

We can calculate the sum using a really handy formula. 

$\frac{a}{1-r}$ where a is the first number and r is the common ratio. 

Plugging in the values we already now, we get that the sum is equal to

$.4 / [ 1 -.09] = .4 / .91 = 40 /91$

So our final answer is 40/91

Thanks! :)