Let's convert this fraction to an equal fraction which holds a denominator of 100, since it is a power of 10.
Since the denominator can be multiplied by 5 to achieve 100, we can multiply the entire fraction by 5 to create an equivalent fraction that has a denominator of 100: 17/20 = (17*5)/(20*5) = 85/100
To obtain a decimal from this fraction, take the numerator and move its decimal one place to the left for every 0 in the denominator. Since our denominator has two zeros, we will need to move the decimal place on the 85 two places to the left.
This gives us an answer of .85, which can also be expressed as 0.85.
We assume the opposite and then get to a point where we get a false statement. That proves that the the opposite of the opposite (What we originally wanted to prove) is true.
