book Maths Puzzle Numerical Ability

I've forgotten how to go about adding and subtracted negatives numbers. Can you help me with what rules to follow? Given the following eight problems:

+25 - (+10)
+25 - (-10)

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book Other Question

Can someone help me simplify the following problems

(-3x / 2y^3)^-2

and

(x^1/3 / x^5)^3

thanks QUESTION: Conor is scuba diving at a depth of 52.8 feet below sea level. Thiry mins later, he is at a depth of 40 feet below sea level. What is his change in depth? Did he go up or down?