That person is WRONG, as usual! This is NOT an A.P. with 4 as the common difference. It is in fact a G.P. with r or the common ratio being 1.5.

You can use the formula for a specific term in a G.P. to find the last term, or (), where: r is 1.5, and a1, or 1st term is 16.

Last term: 

Sum of a G.P.: , where: n = 7     ;      a1, or 1st term = 16     ;     an, or LAST term = 182.25      ;       r = 1.5

Sum of the 1st 7 terms of this G.P.:  

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