Answer:
An = 0.9A(n-1) + 40
Explanation:
10% decrease means 100-10 = 90% of the previous
A1: 1000
A2: (90% of 1000) + 40 = 0.9(1000) + 40
A3: 0.9(0.9(1000) + 40) + 40
= 0.9²(1000) + 0.9(40) + 40
A4: 0.9(0.9²(1000) + 0.9(40) + 40) + 40
An = 0.9A(n-1) + 40