If 3n zeros are placed between the digits 3 and 7, then the number formed is divisible by 37. In addition, if 3n+1 zeros are placed between the digits 7 and 3, the number formed is also divisible by 37.

It means 3000...7 is divisible by 37. Here,the zeroes are in 3n form.

It means 70000...3 is also divisible by 73. Here zeroes are in 3n+1 form.