Explanation:
13.8% = 0.138 in decimal form.
Percent means 'per 100'. So, 13.8% means 13.8 per 100 or simply 13.8/100.
If you divide 13.8 by 100, you'll get 0.138 (a decimal number). Read more below on this webpage.
As you can see, to convert from percent to decimal just divide the percent value (13.8) by 100, and remove the "%" sign.