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## Numbers -> IMPORTANT FACTS AND FORMULAE

 1. Natural Numbers : Counting numbers 1, 2, 3, 4, 5, .. are called natural numbers. II. Whole Numbers : All counting numbers together with zero form the set of whole numbers. Thus, I. 0 is the only whole number which is not a natural number. II. Every natural number is a whole number. III.Some Important Formulae : I. ( 1 + 2 + 3 + .....+ n) = n (n + 1 ) / 2 II. (1 2 + 22 + 32 + ..... + n2) = n ( n + 1 ) (2n + 1) / 6 III. (1 3 + 23 + 33 + ..... + n3) = n2 (n + 1)2 / 4

## Numbers -> SOLVED EXAMPLES

1. 217 × 217 + 183 × 183 = ? Sol. Let the speed of the motorboat in still water be x kmph. Then, Speed downstream = (x + 2) kmph; Speed upstream = (x - 2) kmph. ∴ 6/x+2 + 6/x-2 = 33/60 ⇔ 11x² - 240x - 44 = 0 ⇔ 11x² - 242x + 2x - 44 = 0 ⇔ (x - 22) (11x + 2) = 0 ⇔ x = 22. Hence, speed of motorboat in strill water = 22 kmph. A man can row 18 kmph in still water. It takes him trice as long to row up as to row down the river. Find the rate of stream. Sol. Let man’s rate upsream be x kmph. Then, his rate downstream = 3x kmph. ∴ Rate in still water = 1/2(3x + x) kmph = 2x kmph. So, 2x = 18 or x = 9. ∴ Rate upstream = 9 km/hr, Rate downstream = 27 km/hr. Hence, rate of stream = 1/2(27 - 9) km/hr = 9 km/hr. A man can row upstream at 7 kmph and downstream at 10 kmph. Find man’s rate in still water and the rate of current. Sol. Rate in still water = 1/2 (10 + 7) km/hr = 8.5 km/hr, Rate of current = 1/2 (10 - 7) km/hr = 1.5 km/hr.

## Numbers -> Exercise

16. The largest natural number by which the product of three consecutive even natural numbers is always divisible by

• A. 16
• B. 24
• C. 48
• D. 96
 Ans: C. Sol. Required number = (2 x 4 x 6) = 48.

17. In doing a division of a question with zero remainder, a candidate took 12 as divisor instead of 21. The quotient obtained by him was 35. The correct quotient is

• A. 0
• B. 12
• C. 13
• D. 20
 Ans: D. Sol. Dividend = (12 x 35) = 420. Now, dividend = 420 and divisor = 21. ∴ Correct quotient = 420 / 21 = 20.

18. What least number must be subtracted from 13294 so that the remainder is exactly divisible by 97?

• A. 3
• B. 5
• C. 7
• D. 9
 Ans: B. Sol. On dividing 13924 by 97, we get remainder = 5. ∴ Required number to be subtracted = 5.