4. How many four-digit numbers formed only of odd digits are divisible by five?
4. 125 The number will be of the form ' $a b c d$ ' where $a, b$ and $c$ are any odd digits and $d=5$. Hence there are $5,5,5$ and 1 possibilities for $a, b, c$ and $d$ respectively. Therefore there are $5 \times 5 \times 5 \times 1=125$ such numbers.