2/5 is a simplified fraction, and one of the bigger fractions it’s made by 4/10. Do you can use 4/10, but can also use 8/20, 20/50, 40/100, so on. Since we know that any fraction that of ten can be turned into a percentage easily (2/10=20%, 5/10=50%,9/10=%90), then 4/10 is equal to 40%. So 40% of the people in Jeremiah’s class are boys.
