



 
Hi Ata, Some algebra helps solve this. Suppose your cost is $\$C$ and the selling price is $\$S,$ your profit is then you want your $\$S  \$C.$ You want your profit to be 20% of your selling price so what you need is \[\$S  \$C = 0.20 \times \$S.\] Add $C to each side to obtain \[\$S = 0.20 \times \$S + \$C\] and then subtract 0.20 × $S from each side to get \[\$S  0.20 \times \$S = \$C.\] Thus \[0.80 \times \$S = \$C\] or, dividing both sides by 0.80 \[\$S = \frac{\$C}{0.80}.\] Hence for the example you sent, if the cost is $\$C = \$100$ then \[\$S = \frac{\$100}{0.80} = \$125.\] I hope this helps,  


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