Hi Laura,

If you take a two digit number and insert two zeros between the digits you have a four digit number with a double zero in the middle. Conversely if you take one of your 4 digit combinations and remove the double zeros in the middle you have a two digit number. Thus the number combinations possible with a 4 digit combination with double zero in the middle s exactly the same as the number of two digit numbers. How many two digit numbers are there?

Penny