For scientific notation, your base has to be between 1 and 10, so you have to multiply or divide by factors of ten until you get to the proper base then add an exponent for the number of times you multiplied by 10... Basically, you move the decimal then add the exponents for the number of places it moved. For your number...
0.0000000405 you move the decimal 8 times to the right so it reads
4.05
now you add the exponents. Since you moved the decimal to the right it will be a -8 So you end up with
4.05 × 10^-8
