Taking the variable A to be the percentage of households that had cordless phone in 2000, we can set up the equation:
A + (12.9%) A = 86%
where to convert percentage to decimal, divide by 100:
A + 0.129 A = 0.86
Calculating for A:
1.129 A = 0.86
A = 0.86 / 1.129
A = 0.7617
or
A = 76.17%
So around 76.17% in 2000.
