Adding and Subtracting Within 1,000

$777$

Hundreds: $3 + 4 = 7$. Tens: $2 + 5 = 7$. Ones: $1 + 6 = 7$.

$321$

Ones: $4 - 3 = 1$. Tens: $8 - 6 = 2$. Hundreds: $5 - 2 = 3$.

$423$

Ones: $8 + 5 = 13$, so $1$ ten and $3$ ones. Tens: $7 + 4 + 1 = 12$, so $1$ hundred and $2$ tens. Hundreds: $2 + 1 + 1 = 4$. The answer is $423$.

$390$

Break a hundred: $630$ is $5$ hundreds and $13$ tens. Tens: $13 - 4 = 9$. Hundreds: $5 - 2 = 3$. So $390$.

$275$

From $125$, $75$ gets to $200$, and $200$ more gets to $400$. $75 + 200 = 275$.

$900$

Ones: $9 + 1 = 10$, a new ten. Tens: $0 + 9 + 1 = 10$, a new hundred. Hundreds: $5 + 3 + 1 = 9$. So $900$.

$363$

Ones: $5 - 2 = 3$. Tens: $4$ tens, and $8$ must go, so break a hundred: $6$ hundreds and $14$ tens. $14 - 8 = 6$. Hundreds: $6 - 3 = 3$. So $363$.

$625$

$250 + 250 = 500$. Then $500 + 125 = 625$.

$400 + 90 + 13 = 503$. One hundred was traded for ten tens, and one ten for ten ones.

Trading a hundred for ten tens, and a ten for ten ones, keeps the amount. Adding the parts back gives $503$.

Can you take $5$ ones from $2$ ones?

He took each smaller digit from the bigger one. The answer is $427$.

$602$ has $2$ ones and $0$ tens. Break a hundred and a ten: $5$ hundreds, $9$ tens and $12$ ones.

Ones: $12 - 5 = 7$. Tens: $9 - 7 = 2$. Hundreds: $5 - 1 = 4$. So $427$. Check: $427 + 175 = 602$.