Here are the decimal numbers converted to fractions:

a. 0.648 = 648/1000 = 81/125

b. 0.62 = 62/100 = 31/50

c. 0.240 = 240/1000 = 6/25

d. 0.90 = 90/100 = 9/10

These fractions represent the decimal numbers in a simplified form.