book Maths Puzzle Numerical Ability

I need help

I am in math 8/7 with pre algerbra and it says to simplify this problem

3x+y-x+y

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Can you please help me with the following problem:
How do you find the least common multiple (LCM) for the following group of numbers? 12, 20, and 35.

Can you break down your answer so I will be able to understand?
Thanks! Boiling water, at 100C is placed in a freezer at 0 degree celsius. The temperature of the water is 6 degree celsius after six minutes. What is the temperature after 2 hours?