a, b, c invers proportionale cu 1/2, 1/7 si 1/5
=> a/2=b/7=c/5
b=7a/2
c=5a/2
abc=1890
a×7a/2×5a/2=a³×35/4=1890
a³=216 => a=6
b=21
b=21c=15
