I'm guessing the sum is supposed to be

Split the summand into partial fractions:


If
, then

If
, then

This means

Consider the
th partial sum of the series:

The sum telescopes so that

and as
, the second term vanishes and leaves us with

Answer:
1/(5^12 * 32^3 * 9^15)
Step-by-step explanation:
5^-12 = 1/(5^12)
32^-3 = 1/(32^3)
9^-15 = 1/(9^15)
1/(5^12) * 1/(32^3) * 1/(9^15) = 1/(5^12 * 32^3 * 9^15)
..........................
Area of the board divided by the time
2 x 3 = 6ft^2
15/6= 2.5mins
20 because there are many more