Answer :
Let x be the number of form letters which Frankie can type in an hour.
We know that Diana can type 100/6 = 50/3 form letters in a hour.
Since they can type 100 letters together in four hours, we have:
[tex]\begin{gathered} 4(x+\frac{50}{3})=100 \\ x+\frac{50}{3}=\frac{100}{4} \\ x=25-\frac{50}{3} \\ x=\frac{25}{3} \end{gathered}[/tex]Therefore, the number of hours needed to Frankie do this job alone is given by:
[tex]\frac{100}{x}=\frac{100}{\frac{25}{3}}=\frac{100\cdot3}{25}=12[/tex]