I’m making a nightly inspection form for work, and one of the tasks will be to fill a chemical tank in one of our roof-cleaning vehicles. I’m trying to find an equation for this so I can put it in my form and allow my employees to plug in how much roof mix is already in the tank, and then get numbers for how much of each component of the roof mix they need to fill the tank completely. In the past I’ve just done this in my head, but now that I’m training others to handle it I can’t seem to get myinternal method to transfer to an equation.

This should be simple, but I can’t recall basic algebra. Help?

Relavent info:

There’s a 90 gallon tank labeled “roof mix”. This is what my guys need to fill up every night.

The proper ratio for cleaning an asphalt shingle roof is 40% Sodium Hypochlorate to 60% water. So 1:1.5. There’s one other chemical involved, but it’s a negligible amount and added by hand so we don’t need to include it in the equation.

The amount of roof mix left over each day is variable. Sometimes the crews use it all, sometimes there’s 10 gallons left, sometimes there are 30 gallons left, etc.

I feel like I’m one of those people in the math books with the word problems.

Oh and because someone will probably ask... This is all being pumped in and nobody will be in contact with any of the chemicals. Even so, full face mask with chemical respirators, rubber boots, nitrile gloves and chemical-resistant clothing are required for my team. Safety!