simple - if x then x calculation

I have a coworker who needs help with a calculation:

If we increased the margin to 70%, what would the cost of the item be?
(margin is currently calculated: (retail-cost)/cost = margin


Item Cost Retail margin
crabcake $1.00 $2.00 50.00%
sandwich $2.00 $3.00 33.33%
cake $1.25 $3.00 58.33%

I was thinking it should be something like if retail =.70, then retail is XX. I just have no idea how to put that into a formula.

Thanks!!!
ds

Retail = Cost / (1-margin)
Cost = Retail x (1-margin)
Margin = (Retail - Cost) / Retail

correct - I have it currently as: (retail-cost)/retail = margin

What formula can I use to determine what the retail would be if the margin was 50% ?