(2m-n)/(m+2n)=5
∴(m+2n)/(2m-n)=1/5
∴3(2m-n)/m+2n-m+2n/2m-n+3
=3×(2m-n)/(m+2n)+(m+2n)/(2m-n)+3
=15+1/5+3
=18又1/5