The Apple iPhone comes with a native Calculator app. It has two modes: standard and scientific. Standard mode allows you to perform basic operations such as addition, subtraction, multiplication and division. Switching to scientific mode enables more functions for complex calculations like parentheses, exponents and pi. For fractions, it's best to switch the Calculator app into scientific mode. Two methods are possible in the app for using fractions within an equation.
Open the Calculator app and switch to scientific mode by holding your phone horizontally to activate the landscape orientation.
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Tap the bottom number of the fraction, the denominator, on the keypad. Then press the "1/x" key. This will convert the entire fraction to decimal form, but only works if the top number, the numerator, is one.
Use the division symbol for all other fractions, since fractions are essentially just forms of division. For instance, if the fraction you are trying to enter is ¾, tap "3," then the division symbol, then tap "4." Then press the equal sign.
Make sure the screen orientation lock on your iPhone is turned off, otherwise you won't be able to switch to scientific mode in the Calculator. Do this by swiping up from the bottom of the display to bring up Control Center, then turn off the screen orientation lock if it's on.