Hi There!
First compare the whole numbers, the number on the left of the decimal point. As you can see they are both the same, now look at the decimals, look at the higher number, so 1.9 is greater than 1.5
Another method a teacher might use is the money method, pretend you have $1.90 and $1.50, $1.90 or 1.9 is greater than 1.5 or $1.50.
Hope This Helped :)