I dont know how much help ill be,  but i had major trouble with this one as well

ofcourse you know the formula:        Loss x             Limit of Insurance                       = your settlement

                                                                        Value of Prop.  x coinsurance %

So if your total loss was 20,000 and the insureds limit is 30,000

There total value of property is 50,000

and normal coinsurance is 80%

20,000  X      30,000                                                                               

               50,000 X 80%

I do the bottom half first so 50,000 x .8 = 40,000

Then divide next 30,000 divided by 40,000 =.75

thenmultiply them together to reach your settlement 20,000 x .75 = $15,000

 Now if you still dont understand i can try and help you out more, but good luck with this