Answer:
[tex]4\ weeks[/tex]
Step-by-step explanation:
we know that
John can create an app in 6 weeks
so
In 1 week can create 1/6 of app
and
Ben can create an app in 12 weeks
so
In 1 week can create 1/12 of app
Together in 1 week can create -----> 1/6+1/12=3/12=1/4 of app
so by proportion
[tex]\frac{1}{(1/4)}\frac{week}{app} =\frac{x}{1}\frac{week}{app} \\ \\x=1/(1/4)\\ \\x=4\ weeks[/tex]