book Maths Puzzle Numerical Ability

i could really use your help in this one! the total of eight times a number and fifteen is thirty-one. find the number. thanks

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book Other Question

1. How do you add two intergers if they have the same sign?

2. How does the absolute value of a number in assigning a sign on the sum or difference of intergers?

3.How do you What is Expanding logarithms and how do i do this problem?
ln((e^(4)y)/(d^(2))