How many terms of the series 3,6,12,.. having sum 381?

Explanation

`a=3`

`r=6/3​=2`

Sum`=381`

Sum=`a(r^n−1)/(r-1)​`
=`381=3(2^n−1)/(2-1)`
​
=`381/3​=2^n−1`

`=127=2^n−1`

`=128=2^n`

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