Variations and Percentages! Help

Question

The question is here in the link: http://postimg.org/image/6uq9f3opn/

The last step I could get was:
Let K1 and K2 Be constant

Z = 1.25(K₁)(X ) + 0.64(K₂)(Y²)

... Thanks for your help!

Answer 1

In joint variation, the quantity varies directly as the product of two or more other quantities.  So the equation for z would be:

 

z = kxy²

 

where k is a constant.  When we increase x by 25%, it becomes 1.25x.  When we reduce y by 20%, it becomes 0.8y.  The equation then becomes

 

z₂ = k(1.25x)(0.8y)²

 

% change = (1 - z₂/z)*100% = {1 - k(1.25x)*(0.8y)²/kxy² }* 100% = 20%