Find the sum of first 20 natural numbers which are divisible by7.​

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1. Answer:

   =7+14+21+28+35+42+49+56+63+70+77+84+91+98+105+112+119+126+134+141

   = 7(1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20)

   = 7 {20(20+1)/2 } •{n(n+1)/2}

   =7×210

   = 1470

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