Convert the fraction to make it have a denominator of 100. How do we do this? Multiply the 25 in the denominator with a number that'll give 100, then multiply the numerator with that very same number.
25 * 4 = 100, so we need to multiply 4 to both the numerator and denominator (according to a rule of fractions).
(21 * 4)/(25 * 4) = 84/100
Now we have the denominator set to 100. Take the numerator, 84, and move its decimal point two places to the left. (The reason it's two is because the number was being divided by 100, which is 10^2. The exponent in the 10 tells you how many places to move the decimal point.)
84. --> 8.4 --> .84
So the decimal of 21/25 is 0.84.