(a+1)/20=(b+1)/21=(a+b)/17=k
=>
a=20k-1 (1)
b=21k-1 (2)
a+b=17k (3)
41k -2 =17k
k = 1/12
3a/(2b)
=3(20k-1)/[2(21k-1)]
=3( 4/6)/[2( 9/21) ]
=21/9
=7/3