Return one of 2 sums based on a cell value

Hello,

I am trying to generate a cell value (D120) from data in my worksheet based on the value of a calculated cell on another worksheet ('Lists, formulas, and values'!G2). If it is <20, I want the value to be calculated as - =sum(D40:D44,D45,D47,D48,D49,D50,D55,D56,D57,D58,D61,D62,D63)/17. If it is 20 or greater, I want it to be calculated as - =-sum(D40:D44,D45,D47,D48,D49,D55,D56,D57,D58,D61,D62,D63)/16 - ( I just took at D50).

Thanks for your help