Question:
The sum of all two digit positive numbers which when divided by 7 yield 2 or 5 as remainder is :
Correct Option: , 4
Solution:
$\sum_{r=2}^{13}(7 r+2)=7 . \frac{2+13}{2} \times 6+2 \times 12$
$=7 \times 90+24=654$
$\sum_{r=1}^{13}(7 r+5)=7\left(\frac{1+13}{2}\right) \times 13+5 \times 13=702$
Total $=654+702=1356$
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