Conditional Formula Help

Hi,

I feel I’m stuck with quite a simple conditional formula (I think), but I just can’t figure it out. 

I have an expenses spreadsheet whereby if an employee enters ‘Y’ into the column, I need ann adjacent column to be able to do a VAT calculation. If they enter ‘N’ then no calculation happens. 

The calculation is also based on the current VAT rate from another cell at the top, which could be what’s causing the confusion, but basically I just want a sum to happen based on a response.

The sum that needs to happen is simply =SUM(H20-J7), but only if ‘Y’ is entered into G20 and so on for G20:G29 and H20:H29.

Can anyone please help?

Thanks