Hello, It is very simples.
Look it:
600 = 2100( 1 - 50%)^×
Division both the sides by 2100
600/2100 = 2100/2100.(1 - 0,5)^×
Simplity the zeros
6/21 = 1.( 0, 5)^×
Simplity by 3 the fracto 6/21
2/7 = 0,5^×
Applying Log on both the sides
Log (2/7) = Log (0,5^×)
Applying Log definitely in Log(0,5^×)
We stay:
Log (2/7) = x.Log(0,5)
Dividing both the sides by Log(0,5)
x = Log(2/7) / Log(0,5)
x = 1,8 years
