To find the sum of the series, we can first simplify each term:

4/1 × 5 = 20
4/5 × 9 = 7.2
4/9 × 13 = 5.7777...

We can see that each term is decreasing. Let's find the pattern:

20, 7.2, 5.7777...

The pattern is that each term is the result of multiplying the previous term by a fraction less than 1. This is a geometric series.

To find the sum of the series, we can use the formula for the sum of a geometric series:

S = a / (1 - r)

where:
S = sum of the series
a = first term
r = common ratio

In this case, the first term (a) is 20 and the common ratio (r) is 4/5.

S = 20 / (1 - 4/5)
S = 20 / (1/5)
S = 100

Therefore, the sum of the series is 100.