Using it to calculate, and to read a situation

This property makes hard products easy. You know 53 × 18 = 954. What is 63 × 18? 63 = 53 + 10, so 63 × 18 = 53 × 18 + 10 × 18 = 954 + 180 = 1134. The hard part was already done.

It works downwards too. 97 × 25: 97 = 100 − 3, so 97 × 25 = 100 × 25 − 3 × 25 = 2500 − 75 = 2425.

Try: 95 × 8, 104 × 15, 49 × 50. Is it quicker than your usual method? It helps whenever a number is just above or just below a round number.

Writing situations as expressions:

• Begur market is open all 7 days. Rahim brings 9 kg of mangoes and Shyam 11 kg each day: 7 × (9 + 11) = 7 × 20 = 140 kg a week.
• Binu earns ₹20,000 a month and spends ₹5,000 on rent, ₹5,000 on food and ₹2,000 on other things: she saves 20,000 − (5,000 + 5,000 + 2,000) = ₹8,000 a month, and 12 × 8,000 = ₹96,000 a year.

Some need careful thinking, not a formula:

• A snail climbs a 10 cm post: up 3 cm by day, down 2 cm at night, so 1 cm a day. But on the day it reaches the top it doesn't slip back. It gets there after 8 days, not 10.
• Melvin reads a story every day except Tuesdays and Saturdays: (7 − 2) × 8 = 40 stories in 8 weeks.