I just go this message

"We're giving away Halloween promo code treats for PhySyCalc for iOS. If you want to share with your readers feel free to post this link."

http://redeemco.com/g/physycalc-scientific-and-engineering-calculator_fpjc/

PhySyCalc reinvents the calculator. More intuitive than conventional calculator apps, PhySyCalc lets you do full calculations with units. For example, you drive 250 miles on a full 12 gallon tank of gas and want to calculate your mileage. In PhySyCalc you can skip all the unit conversions and just enter 250 mi/12 gal. PhySyCalc gives you the answer in mi/gal. Or, if you like, km/L or L/100 km, all in a few touches of your finger.