Calculations for combining two volumes of wort at different SG

I used to combine different SG of wort all the time, used promash to calculate, on make using different software and doesn't have this included.

Here is the jest of things.

Start sparge and collect 5 gallons of 1.060 wort. Take kettle over to burner and start heating, stick bucket and collect rest 2.25 gallons of 1.014 wort.

Carry over to burner...but usually stop at this point and check with promash if combining these two liquids will get my target starting gravity, if low add some DME, if high add water or some percentage of the final wort.

So, I don't want the answer, I want the math. On paper, how do you add two volumes of different specific gravity and get the resultant specific gravity.

Thanks

Most calculators only show adding water or boiling off.

[Sg(1)*v(1)+sg(2)*v(2)]/(v(1)+v(2))