book Maths Puzzle Numerical Ability

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?

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book Other Question
I need to solve this log problem:



1) log( 8, (n-3)) + log( 8 (n + 4)) =1


I believe that the "logs" cancel out because they both have the same base. Then I dont understandwhat to do from there
 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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