I’m not sure abt 21 but 22 is the second one and the 4th one. So 2,4.
F = 
express the equation with F on the left side
3F - 24 = s ( add 24 to both sides )
3F = s + 24 ( divide both sides by 3 )
F = 
X= 30
I don’t know if this is correct or not
Answer: C
Step-by-step explanation: Because it's the biggest number
Its 1/5,
35/100 - 15/100 = 20/100
for 20, 40, 60, 80, 100
1, 2, 3, 4, 5
so 20/100 = 1/5