How many numbers with ten significant figures satisfy that all of its digits are even?
My attempt:
If I have ten 'cells', the number of functions from the set of cells to the set of even numbers $A=\left \{ 0,2,4,6,8 \right \}$ is the total amount of numbers with digits which are even. Because I want the ten figures to be significant, I have to substract the number of variations with repetition which have one leading zero (the number of functions from the set of cells minus one to the set of even numbers $A$). And so, $$n=VR(5,10)-VR(5,9)=5^{10}-5^9=4\cdot5^9$$
Is all that right?