uang tau jawapin ini yah?

padacermin cembung berlaku :

-1/f = 1/so + 1/si
-1/4 = 1/20 +1/si
-1/4 -1/20 = 1/si
-5/20 - 1/20 =1/si
si = 20/6

m = si/s
    = 20/6/20
    = 1/6 kali