hi,

I need a formula which will add together the total number of units (shown
monthly) and for every 90th unit put a cost of £250.

So, for example:

Month 1 2 3 4 5
Units 45 46 15 76 22
Cost £0 £250 £0 £250 £0

How can I do this without it showing £250 in every month???

Thanks

Amanda