How to cross cancel fractions for example 3/5 •15/18

Here is how you cross cancel these fractions:

We can start by observing that 18 is a multiple of 3, and it is obtained by multiplying 3 by 6.
So... We can cross out 3 and put a 1 in its place, and we can do the same for 18 but instead by putting a 6 in its place.
This is what we have now:

However, we are not done yet, we can see that 15 is a multiple of 5, which can be obtained by multiplying 5 by 3.
Therefore... We can cross out 5 and put a 1 in its place, and we can cross out 15 and put a 3 in its place. This is what we have now:

Now, we can simplify the expression above to get 1/2, or 0.5 in decimal form. Hope this helps and have a nice day!