I'm not sure about solving problems, #1 and #2.
There seems to be different ways to answering these types of problems and the information/question they give varies form problem to problem. Is there a universal setup for these types of problems?
A biologist wishes to have 15 gallons of an 80% formaldehyde solution. In her inventory, she has pure formaldehyde and some 50% formaldehyde solution. How many gallons of each should she mix to obtain the desired solution?
So do I set up the equation 2x+.50(15-x)=15(.80)
so x=3 gallons? ,15-3= 12 gallons
A leather finisher determines that his 8-ounce solution of 12% boric acid is too weak. How many ounces of pure boric acid must be added to increase the concentration to 20%
15*(.12)which would equal: .96+x. Does that express what is needed to increase the boric acid and where does the 20% come in to play?