I tend to just imagine the graph than to write it in paper. I do the planning first in my mind before doing the work on paper. This is something I cannot do at 3 am now.
Even straight out 0.5 as values for both a and b could work, I guess.They will add up to a number greater than 12.5. In fact, I know that any decimal numbers that a and b will add into one will produce a greater number than 12.5. That, I am well aware.
What does one mean by "It's the only way you know"?
Have your tried it in algebriac calculation already?
I guess I already have a grip on your solution... but the aim of me looking about is to see if it can be solved by algebriac means. Just to see a clearer form in which one can explain to an even slightly mathematically inclined person so he/she could understand.