As its one significant figure you are looking at the first number and the second number. If the second number is greater than 5 then you round the first number up and vice versa. In this case there is an eight, which is bigger than 5 obviously so you round the first number to get 40000.
Answer:
8 years
Step by step guide:
100,000 x 8 = 800,000
600,000 x 8 = 4,800,000
6,900,000 + 4,800,000 = 11,700,000
12,500,000 - 800,000 = 11,700,000
