Question 193489
Say you used the inventions on a tank that holds
{{{100}}} gallons of fuel. With the 1st invention,
you'd only use 70 gallons to go a far as you went 
on a tankful before. Call that distance {{{d}}} mi
With the 2nd invention, you'd use 55 gallons
With the 3rd invention, you'd use 75 gallons
The fuel efficiencies are:
No inventions: {{{d/100}}} mi/gal 
1st invention: {{{d/70}}} mi/gal
2nd invention: {{{d/55}}} mi/gal
3rd invention: {{{d/75}}} mi/gal
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These are RATES of fuel usage. I can add
the rates to get a combined rate
{{{d/70 + d/55 + d/75}}}
{{{d*(1/70 + 1/55 + 1/75)}}}
{{{d*(165/11550 + 210/11550 + 154/11550)}}}
{{{d*(529/11550)}}}
{{{d*.0458}}}
{{{d*(1/21.834)}}}
{{{d*(1/70 + 1/55 + 1/75) = d/21.834}}} mi/gal
That's how much of the 100 gallon tank
you'd use to go {{{d}}} mi so you would
save {{{100 - 21.834 = 78.166}}} gallons
{{{78.166/100}}} = 78.166% saved
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I can check this by converting to decimals:
{{{d*(1/70 + 1/55 + 1/75) = d*(.04286 + .01818 + .01333)}}}
{{{d*.04579}}}
{{{d*(1/21.836)}}}
Close enough