I see three (3) question in the picture.
Here are the answers:
#1).
The total effective resistance of resistors in series is
the sum of the individuals.
(4 + 8 + 12) = 24 ohms.
Current = (voltage) / (resistance) = (24 V) / (24 ohms) = 1 Ampere.
#2).
To answer this question, we don't care what the
total effective resistance is.
The resistors are in parallel, so each one is connected
directly to the terminals of the battery, and each resistor
has 24 V across it.
Current through the 4-ohm resistor = (voltage) / (resistance)
= (24 V) / (4 ohms)
= 6 Amperes.
#3).
In a series circuit, the total effective resistance is
the sum of the individual resistors, and the current
is (battery voltage) / (total effective resistance).
If pone resistor is replaced with a resistor of lower resistance,
then the sum of the individuals is less, and the current in the
series circuit increases (if the battery can handle it).