An easy way that I use to use to get percentages was get the denominator to equal 100. Anything over 100 would be the percentage, that's the way I was taught.
<em>You multiply by to get the denominator, 20, to equal 100</em>
We take the total profit (58300) and subtract the value of all 3 contracts to see what is left over. That remaining amount (14850) is the final contract.