hiii guys minta bantuanya yah.. please answer, thx u❤

s = 20 cm
s'= 60 cm
f = ?

Penyelesaian:
1/f = 1/s + 1/s'
1/f = 1/20 + 1/60
1/f = 3/60 + 1/60
1/f = 4/60
f = 60/4
f = 15 cm