book Maths Puzzle Numerical Ability

How to solve linear systems in 4 varibles.
for example:
W + X +Y + Z = 3
W - X + Y + Z = 1
W + X - Y + Z = 1
W + X + Y - Z = 3
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book Other Question

I do not understand how to solve this equation n-15=5/2*n, I do not know which section to solve first can you please help me? in words, write out the steps necessary to solve for x in each of the following equations if you were given the values for the other variables.

x=tb
x=y tho the 3rd power+w to