Question

Find the sum

(a) 1 + 2 + 3 +.......................................+ 20

(b) 3 + 6 +. 9 +..................................... + 60

(c) 5 + 8 + 11 +.....................................+ 62​

Answer 1

(a) 1 + 2 + 3 +.......................................+ 20= 210

(b) 3 + 6 +. 9 +..................................... + 60= 630

(c) 5 + 8 + 11 +.....................................+ 62= 670

Answer 2

Answer:

A. Sum of first and last digit = 1+20 = 21

It will form 10 pairs. So,

Sum = 21 × 10

= 210

B. Sum of first and last digit = 3+60 = 63

It will form 10 pairs. So,

Sum = 63 × 10

= 630

C. Sum of first and last digit = 5+62 = 67

It will form 10 pairs. So,

Sum = 67 × 10

= 670

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