book Maths Puzzle Numerical Ability

if a = 5b
then 3/5(a) = ?

the answer is 3b but I don't understand the rationale behind it, please explain!?

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book Other Question

If the perimeter of a rectangle is 40 inches, find the dimensions of the rectangle. X is the length across and x-4 down each side. Please help! Can you please help me solve:
6x+28(4x-7)=9x