Open-economy private saving identity Steps:

• Public saving equals budget surplus: $20bn.
• Net exports NX equals negative trade deficit: -$20bn.
• Investment I is $50bn.
• Private saving Sp = I + NX - budget surplus = 50 + (-20) - 20 = $10bn.
• Total saving refers to private saving Sp.

Why B is correct:

• The identity Sp = I + NX - (T - G) yields $10bn, matching private saving as total saving.

Why the others are wrong:

• A: Subtracts budget surplus twice, ignoring NX adjustment.
• C: Equals national saving S = I + NX = $30bn, excluding public saving subtraction.
• D: Matches budget surplus alone, ignoring I and NX.

Final answer: B