To calculate how many landlines will be used in 7 years you have to apply the exponential decay
[tex]y=a(1-r)^x[/tex]Where
a is the initial value
r is the decay rate (this value is given as a percentage, you have to use it expressed as a decimal)
x is the time interval that has passed
We know that there are 42000 landlines this year
The declining rate is 10% → expressed as a decimal value r=0.1
The time-lapse is 7 years
[tex]\begin{gathered} y=42000(1-0.1)^7 \\ y=20088.47 \end{gathered}[/tex]In 7 years there will be 20088.47 landlines
