Answers

2014-03-12T12:39:42+05:30
Let the no be a-6d,a-5d,a-4d,a-3d,a-2d,a-d,a,a+d,a+2d,a+3d,a+4d,a+5d,a+6d
so 
13a/13=324
a=324
now for 20
20a/20=a=324
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2014-03-15T08:39:22+05:30
(x+x+1+x+2...+x+11+x+12)/13=324
13x+78=324*13
13x=4212-78
13x=24134
x=318
average of first 20 consecutive no.=(x+x+1+..+x+19)/20
(20x+190)/20=(20*318+190)/20
10(636+19)/20=655/2
327.5
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