(20/3)^a×(8/15)^b×(9/16)^c=4
[(2^2×5)/3]^a×[(2^3)/(3×5)]^b×[(3^2)/(2^4)]^c=4
[2^(2a+3b)×3^2c×5^a]/[2^4c×3^(a+b)×5^b]=4
2^(2a+3b-4c)×3^(2c-a-b)×5^(a-b)=2^2
∴2a+3b-4c=2
2c-a-b=0
a-b=0
a=b=c=2