2.9 Understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

In a series circuit, current is the same everywhere. This means that it can be worked out using the equation I = V/R . This means it is the total voltage supplied divided by the total resistance of all the components. 

Eg. if a battery supplies the circuit with 30V, what is the current? There are 3 bulbs and each has a resistance of 2, 4 and 7 ohms respectively.

So... let's name the bulbs. There are three. Let's say that...
Nimue bulb = 2 ohms
Esyae bulb = 4 ohms
Morthil bulb = 7 ohms

Nimue + Esyae + Morthil = 13 ohms total resistance
    (2)           (4)        (7)

The total voltage, we already established, is 30 V.

30 / 13 = 2.30769231
Round off to 3 s.f. or 2 d.p.
= 2.31 (3 sf)