MySql | How to compute this I have this table

Grade        Lec         Lab
1.5           3           0
1.5           3           0
1.5           2           1


The formula will be like this

1.5 * 3
1.5 * 3
1.5 * (2+1) = 13.5 / 9 = 1.5


13.5 is the total then 9 is the total sum of the lec + lab 

What query should I use?

How can i add this to query WHERE Lec + Lab = 15. If lec + lab is 15 above will show but if lower than 15 it will not show then i want to limit the show in 20
SELECT SUM(Grade * (Lec + Lab)) / SUM(Lec + Lab) FROM t;




| SUM(Grade * (Lec + Lab)) / SUM(Lec + Lab) |
| ----------------------------------------: |
|                                       1.5 |



db&lt;>fiddle here