Question 876489
completing pay roll = 1 job
let new computer take x hours to complete the job
New computer does 1/x of the job in 1 hour
Old computer does 1/(x+6) of the job in 1 hour

Together they do 1/4 of the job in 1 hour


1/x+ 1/(x+6) =1/4

solve

{{{((x+6)+x) /(x(x+6))=1/4}}}

{{{4(2x+6) = x^2+6x}}}

{{{8x+24=x^2+6x}}}

{{{x^2+6x-8x-24=0}}}

{{{x^2-2x-24=0}}}

{{{x^2-6x+4x-24=0}}}

x(x-6)+4(x-6)=0

(x-6)(x+4)=0
x=6 OR x-4

ignore negative

new computer takes 6 hours
Old takes 6+6 =12 hours