Break numbers into friendly parts
For addition and subtraction, bridge through tens or hundreds. Instead of holding 47 + 38 as one operation, use 47 + 30 + 8 or 47 + 40 − 2.
For multiplication, distribute: 16 × 7 becomes 10 × 7 plus 6 × 7. The method reduces working-memory load and makes errors easier to spot.
Estimate before calculating exactly
A quick estimate gives the answer a neighbourhood. If 49 × 21 is close to 50 × 20, a final answer near one thousand is plausible; one near ten thousand is not.
Estimation is a safety check, not a substitute for the exact answer when precision is required.
Train accuracy, then shorten the time
Practice one technique in small sets and record both correct answers and time. Increase speed only after the process is stable.
Review the type of error: lost sign, wrong carry, rushed multiplication or misunderstood question. Correcting the process is more useful than repeating random calculations.