Let's pause here.

For all proportion problems, start with this:

Varying           "directly" or product of "jointlys" or 1 if none 
quantity  = k ∙ ----------------------------------------------------------
                inversely variable or product of "inverselys" or 1 if none

In this problem the varying quantity is F. There are no "directly" or "jointly"
variable.  So the numerator is the number 1.  We have one inversely, d2. So we
have F on the left, 1 on the top of the fraction and d2 on the bottom:



-----------------------

Let's pause again here.
Let's substitute 30 for F in , and 5 for d2>/sub>, and solve for k:



Put 1 under 30 and write 52 as 25:



Cross-multiply:



Now we substitute 750 for k in 







 find F when d = 12 meters.





 newtons.

Edwin