math

can anyone help please, i need a simple formula for working out taxes.if i had any number i need to know what the base was before 16% tax was added. i know the formula if the tax was 17.5%,what ever the total is if you divide that by 47 then x that by 7 that will give you the base before tax was added. example 50 divide 47 x 7 = 7.44 this gives me the total tax i paid if the rate was 17.5% , i need to know a similar formula if the tax is 16%,thanks. why bother with a "formula"? If you have the total after 17.5% tax was added, that total must represent 117.5% of the original. So if you divide your total by 1.175, which is 117.5%, you get the original. Now multiply that by .175 and you have the amount of the tax. so $50÷1.175x.175 = $7.45 (notice 1.175/.175 = 1175/175 = 47/7 ) if you had 16% your calculation would be $50÷1.16 x .16 = ..... .16/1.16 = 4/29 So your formula would be "divide by 29 then multiply by 4" for 12% it would be .12/1.12 = 3/28 etc. Personally, I would never use such a "formula", just do the math by reasoning it out.