book Maths Puzzle Numerical Ability

hi kim when you see an algrebric expression how do uyou know what number to add or subtract to both sides.

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book Other Question
Write 0.8(with a line over the 8) as a fraction in lowest terms. How do I do this? Thank you. I am having trouble with this question, Find the Lowest Common Denominator of

3 7
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x^3 xy^4

I think the answer is

xy^4

Am I right, Can someone let me know?