The number of cell phones in the United States was 1.2 million in 1995. if the annual increase in the number of phone per year is 21%. In what year will there be 1 billion cell phones in use

Answer:Year 2031Step-by-step explanation:We need to use the formula for compound growth here, that is:[tex]F=P(1+r)^t[/tex]WhereF is future amount (1 billion)P is present amount (1.2 million)r is rate of growth (21% or 0.21)t is the time (we need to find this)Putting in the values we have:[tex]F=P(1+r)^t\\1,000,000,000=1,200,000(1+0.21)^t\\833.33=(1.21)^t\\Ln(833.33)=Ln((1.21)^t)\\Ln(833.33)=t*Ln(1.21)\\t=\frac{Ln(833.33)}{Ln(1.21)}\\t=35.28[/tex]So after about 35.28 years, the number of phones will cross 1 billion, so we take the next year, so after 36 years.From 1995 to 36 years would be the year 2031