math30
posted by alejandro on .
A security code used to consist of two odd digits, followed by four even digits. To allow more codes to be generated, a new system uses two even digits, followed by any three digits. If repeated digits are allowed, the increase in the number of possible codes is____
Answer: Old code: 5x5x5x5x5x5=15325
New code: 5x5x10x10x10=25000
The increase in the number of codes is 9375.
I don't understand why they use 5's and 10's!!!Please somebody can explain me this!!!

how many even digits are there from 0  9 ??
there are 5
how many digits are there from 0 to 9 ?
there are 10
so you have 5 choices as the first digit, 5 choices again for the second digit, 10 choices for the 3rd digit, etc 
Thanks, but what about the odd number?

let's see. You know there are 10 digits
You know there are 5 even digits.
asking now how many odd digits there are is the same as holding out both hands, and when someone says, "This one is your left hand," you ask, well, which one is my right hand?