The missing exponent is 5^7 your welcome
50 = 1/2 (20+ x)
100 = 20 + x
80 = x
answer
x = 80
This is pretty obviously

If it wasn't obvious we can find the common ratio r by dividing successive values:
5/1=5
25/5=5
125/25=5
625/125=5
That's the common ratio. We multiply by the first term, but the first term is one, so that has no effect.
Answer:
This means that the equation has infinitely many solutions.
Step-by-step explanation:
Say I have a problem like 1+x=1+x.
Subtracting 1 from both sides gives me x=x.
Any number can be x b/c any number equals itself.
You can prove this by checking: 4=4; 1=1; 1000=1000; -356=-356; 
Any number will equal itself, so the equation has infinitely many solutions.