Find the LCM.
5, 20, 68

to add: align your decimal and add just like you would regualar 2 or more digit numbers

To subtract: Write down the two numbers, one under the other, with the decimal points lined up. Add zeros so the numbers have the same length. Then subtract normally, remembering to put the decimal point in the answer.

To multiply: Multiply normally, ignoring the decimal points. Then put the decimal point in the answer - it will have as many decimal places as the two original numbers combined