Q.The following errors were detected before the preparation of trial balance. Rectify them.

Both are one-sided casting errors found before the trial balance, so each is set right by correcting the single affected account — credit Sales A/c ₹100, and credit (reduce) Purchases A/c ₹400.

These errors are located before the trial balance is prepared, so no Suspense Account exists yet; each one-sided error is rectified by directly adjusting the affected nominal account.

1. Sales book undercast by ₹100 The sales book total was carried to the Sales Account short by ₹100, so Sales A/c was credited less by ₹100. To rectify, credit the Sales Account with ₹100. Rectification: enter on the credit side of Sales A/c — "By Undercasting of sales book ₹100".
2. Purchases book overcast by ₹400 The purchases book total was carried to the Purchases Account in excess by ₹400, so Purchases A/c was debited excess by ₹400. To rectify, credit the Purchases Account with ₹400. …

Each error is a wrong casting of a returns book that affects only one account, so it is rectified by adjusting that account's balance directly (before the Trial Balance, no suspense account is needed).

Undercast = totalled short; overcast = totalled in excess. Purchases Returns is a credit-nature account; Sales Returns is a debit-nature account.

These are one-sided casting (totalling) errors found before the trial balance, so each is rectified by correcting the total of the concerned subsidiary book directly — no journal entry and no suspense account.

Before the trial balance is prepared, a one-sided error (an error affecting only one account/book, such as wrong casting) is corrected simply by adjusting the total of the relevant book — you write an explanatory note against that total. 'Undercast' means the total is short and must be increased; 'overcast' means the total is excess and must be reduced.