



 
Hi, Some algebra shows what to do. If the contract amount is $\$C$ then since $0.70 \times 0.05 = 0.035,$ the PST is
Hence the subtotal, which I'll call $S is
This is a one step calculation to find the subtotal $S,
But you have $\$S$ and you want $\$C$ so in the equation above divide both sides by 1.035 to get
Thus for the example you sent where $\$S = \$103.50,$
I hope this helps,  


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