Article in English: Let's start with a new example. The world population went from 5,300,000,00 people in 1990 to 7,400,000,000 in 2015. There's a trick to these problems with many zeroes. Instead of counting the zeroes each step of the way, we can rewrite these as 5.3 billion and 7.4 billion. This will tell us how much bigger the end result is than the original.  7.4 billion ÷ 5.3 billion = about 1.4. We've rounded to two significant digits because that's how many there were in the original problem. This will tell you the percentage comparison between the two values. If the value increased (instead of decreasing), your answer should always be larger than 100. 1.4 x 100 = 140%. This means the world population in 2015 is 140% the size of the population in 1990. In this kind of problem, "100%" is the size of the starting value. By subtracting this from our answer, we're left with just the percentage size of the increase.  140% - 100% = a 40% increase in population. This works because starting value + increase = end value. Rearrange the equation and we get increase = end value - starting value.

Summary in Vietnamese:
Viết ra giá trị đầu và giá trị cuối. Lấy giá trị cuối chia cho giá trị đầu. Nhân cho 100. Trừ cho 100.