Excel Formula Needed

[mwmnaqv0]

okay first of all I apologize if this is the wrong forum to ask this question or if it has been answered already. I am trying to create a formula in excel spread sheet to calculate a specific result for example

Salesman A sold 23 TVs 20 out of 23 TVs were sold with a HDMI cable, which comes out to roughly 87% HDMI attachment rate. How many more TVs with a HDMI cable does he/she need to sell in order to achieve a rating of 90%. answer is "7" he/she needs to sell 7 more TVs with a HDMI cable which would put him/her at 30 total TVs sold and 27 of which include a HDMI cable which is 90% of 30. what formula would I use to have the excel generate this answer for me automatically or am I just expecting too much from MS Excel. I am attaching a sample file with some of the values entered manually just so you guys know what i am trying to accomplish. Thanks in advance.

[royUK]

This might be what you want