Entering text in one cell, affects the value of another

[The1theycallp]

Hi I'm new to this forum so please be kind and help me.
I want to create a cell which when a "Y" is entered, I want it to half the value of another cell which is using a lookup function.

To clarify, cell I4 has a vlookup function which converts a percentage in H4. into minutes of downtime within an hour. I want to add a "Y" which will indicate half the time worked, to cell C4 which will half the minutes downtime shown in I4

Any help would be greatly appreciated.

[martindwilson]

change the formula with in i4 with the vlookup to
=if(c4="y",vlookupformula/2,vlookupformula)
or =vlookupformula/if(c4="y",2,1)