78.4% is equivalent to 784 parts out of 1000 or 784 / 1000.
Find the greatest common divisor (GCD) of these two numbers and divide the numerator and the denominator by the GCD.
GCD Algorithm
1000 / 784 = 1 with remainder 216
784 / 216 = 3 with remainder 136
216 / 136 = 1 with remainder 80
136 / 80 = 1 with remainder 56
80 / 56 = 1 with remainder 24
56 / 24 = 2 with remainder 8
24 / 8 = 3 with remainder 0
The last remainder before 0 is the GCD which is 8.
784 / 8 = 98
1000 / 8 = 125
Hence 98 / 125 is 78.4% to a fraction reduced to lowest terms.