book Maths Puzzle Numerical Ability

A CERTAIN BRASS ALLOY CONTAINS 60% COPPER, 34% ZINC, 6%LEAD. HOW MANY KG OFTHIS BRASS ALLOY AND OF ZINC MUST BE ADDED TO 20KG OF LEAD TO PRODUCE A NEW BRASS ALLOY THAT IS 40% ZINC

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I have no idea if i solve this in a (a + b)(a - b)=a^2 - b^2 way.

But the problem is to simplify in simplest form:

(The squrt of 3 minus the squrt of 5)*(The squrt of 3 minus squrt of 5) (8+7)(4)=n(4) what is the property used?