Di and Leo both write the decimal 2.55555555 as a fraction.

Di’s fraction: 2 55/100
Leo’s fraction: 2 55/99

Who is correct and why?

Leo is correct.

To convert the decimal 2.55555555 to a fraction, we can set up the equation:
x = 2.55555555

Multiply both sides by 10 to eliminate the decimal:
10x = 25.5555555

Next, subtract the original equation from the equation multiplied by 10:
10x - x = 25.5555555 - 2.55555555
9x = 23

Simplify:
x = 23/9

This means that the fraction form of 2.55555555 is 23/9, which matches Leo's fraction. Di's fraction of 55/100 is incorrect because it does not simplify to the decimal 2.55555555.