How many numbers lie between 300 and 500 in which 4 comes only one time ?

The numbers between 300 and 500 in which 4 comes only one time will start with either 4 or 3 and will be a 3-digit number.

Case I: first digit is 3.

So, either the tens or units digit can be 4.

If tens digit is 4, the units digit can be filled in 9 ways.

If units digit is 4, the tens digit can be filled in 9 ways.

So, there are 18 numbers starting with 3and having exactly one 4

Case II: first digit is 4

The tens digit can be filled in 9 ways and for each of these ways the units digit can be filled in 9 ways.

So, there will be 9 x 9 = 81 such numbers

Total numbers = 81 + 18 = 99