j=10
if you dont need an explanation dont read this:
firstly you reduce the 4/18 by 2 (basically simplifying) , which then becomes 2/9..
then you cross the 2 by 45 and 9 to j (you multiply 2x45 and 9xj=9j)
so it equals to 90=9j
then you divide both sides by 9
j=10