Write 3 x 10^-5 as an ordinary number

 the answer I got is 3/100000

X10^-5 is the same way as saying divided by 10^5
10^5 is 1 with 5 zeroes on i.e. 100000

Therefore 3x10^-5 = 3/100000 = 0.00003

(A quick way of doing this is that the number when there is a minus sign is also the decimal that the first significant figure will be at, i.e. for 3x10^-5 the 3 is the 5th decimal after the 0, but it's good to understand the fundamental principle of it)

It is important because sometimes ur dna needs to multiply