When resistors are connected in series, they act like
a single resistor whose resistance is their sum.
100 ohms and 400 ohms, connected in series, look like
a single resistor of 500 ohms.
Current = (voltage) / (resistance)
= (60 volts) / (500 ohms) = 0.12 A.
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Current is measured by connecting a meter in series
with an energized component. In other words, a break
is made in the circuit, the meter is connected in the break,
and the current to be measured literally flows through the meter.