write all possible 3 digit numbers the sum of whose digits is 4.

Answer 1

103
(1+ 0 + 3 = 4)
Any combination/order you write will work.

112
(1+1+2=4)
Any combination/order you write will work.

220
(2+2+0=4)
Any combination/order you write will work.

400
(4+0+0=4)
Only this combination/order will work.

You should have a total of 10 numbers.

Try listing them all here.

Answer 2

Typo:

For any combination, 0 cannot be the first number.