A Large Sum Including 7

Number Theory Level 2

What is the tens digit in the sum above?


The answer is 5.

Victor Paes Plinio by Victor Paes Plinio , 21, Brazil

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4 solutions

Harsh Poddar
May 2, 2014

first add 77times 7 get 539 9 ones digit 53 carry then add 7 76 times because first number 7 has no tens we get 532 add carry 532+53=585 so tens digit=5

3 Helpful 0 Interesting 0 Brilliant 0 Confused
Ratul Pan
Oct 19, 2015

for unit's digit is=7x77=539
therefore units digit is=9
tens digit =7x76+53(carry over)=532+53 =585
therefore tens digit is 5

2 Helpful 0 Interesting 0 Brilliant 0 Confused
Jonah Robotham
Nov 29, 2014

Start with: 7+77+777 +...+ 77..77 Only the ones and the tens columns can contribute to the tens column in a sum so: 7+77 + 77+---+77 = 7 + 77*76 = 5859, so the tens digit is 5.

0 Helpful 0 Interesting 0 Brilliant 0 Confused
Sunil Pradhan
Jul 14, 2014

unit digit sum = 77 × 7 = 539

10's digit sum = 76 × 7 × 10 = 5230

5230+539= 5859

10's digit = 5

0 Helpful 0 Interesting 0 Brilliant 0 Confused

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