(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*5^a/3^a)*(2^3b/3^b*5^b)*(3^2c/2^4c)=4
2^(2a+3b-4c)*5^(a-b)*3^(2c-a-b)=2^2
a-b=0,2c-a-b=0,2a+3b-4c=2
a=b,2c-a-b=0,2a+3b-4c=2
2c-a-a=0,
2c-2a=0,
a=c=b
2a+3b-4c=2
2a+3a-4a=2
a=2
a=b=c=2
abc=2*2*2=8