Answer:
The answer is

Step-by-step explanation:
Let The number is x
Then the equation is

6 robins is the answer, because 16-10=6
First term, a
1
=4
Second term, a
2
=8
Common difference, d=a
2
=a
1
d=8−4=4
∴ The common difference is 4
This would be a depreciation formula
which is value = Original price x (1-percent change)^time
using the information from the problem
Value = 400 x(1-0.13)^6
value = 400 x 0.87^6
value = $173.45