Percentage Formula Assistance

Can someone take a look at the attached sheet and see if you can help me come up with the formula. I think the sheet has all the info you'd need, but just in case:

Tax is 7%

What I want to happen is to input a total cost, and the sheet breaks down the Principal & Tax amounts. Ex. $75 is cost of principal & tax total... Principal is $70.09, tax is $4.91. I can do the formulas on row 2 obivously, but it's not exactly what I need.

All help appreciated.

Thanks... you guys always come through.