My Casio fx-300ES calculator has a problem when i press the fraction button under Abs i get a symbol looking like "_l" anyway to fix this?

To see a stacked fraction, set the Input/Output mode to MathIO. Press [SHIFT][Mode] (SetUp).

To convert a single value between the symbolic and decimal formats, press the S--D key (above the DEL key). To have all results come out in symbolic form press SHIFT [MODE] 1 to select the MthIO mode.

Be aware that the calculator cannot display all values in symbolic form.

To convert a single value from symbolic to decimal, press the S-D key (just above the DEL key). To have all results come out in decimal, press SHIFT MODE 2 to switch to the LineIO mode.
You can download the manual and the appendix here.

To change a displayed result into a fraction, press the [S=D] key.
To force a result to be displayed as a decimal number before it is displayed, press [SHIF][=] instead of [=].

The calculator has a key marked with two squares on top of one another and with a fraction bar between them. The key is to the left, below the ABS key..
The shifted function of that key is the mixed fraction [a b/c]. I am not convinced you need a specific key to enter a mixed fraction: you can just type in the integer part (whole part) then + then use the fraction key (the one with the two squares.)

To insert pi on this calculator press [shift] [x10x] . (you can see the pi symbol above and to the side of this button).

In math IO mode, press the key marked with a two-level fraction symbol (1 solid square in numerator, fraction bar, 1 empty square as denominator.) A fraction template is displayed. Use the up or down key to move cursor to numerator or denominator. Press [=] key when finished.

To convert a single result to decimal, press the SD button. To have all results come out in decimal, press SHIFT MODE 2 to select the LineIO mode.

Look for button like [ a b/c ] it's the fraction first enter the whole number then press the button then the upper then the button and the down part, if you need only b/c so enter b press the button then enter c.
Rate me, thanks.

just press the button below 'Abs' button.