Answer:
13.) 44V
14.) 66V
15.) 9A
16.) 12V
Explanation:
Concepts you need to know:
1. Resistors in series can be combined by summing up their values.
2. The current always stays the same when the resistors are in series.
3. Ohms Law: 
13.) We know that the resistors are in series so we can combine them into a single big resistor. We got R₄ = R₃+R₂+R₁ = 2+4+5 = 11Ω.
We also know that current is the same in a series of resistors, so there's 4A going through the entire circuit.
Using ohm's law, we can calculate V = IR = (4)(11) = 44V
14.) Same explanation as part (13). Except now the current is I=6A. All we have to do is to replace the previous I=4 to I=6.
V = IR = (6)(11) = 66V
15.) Again, we know that current is the same when resistors are in series. So if you can find the current through R1 then that's the same current throughout the entire circuit.
We're given E₁=18V, so...
I = V/R = E₁/R₁ = 18/2 = 9A
this means 9A will also flow through R₃.
16.) Same concept as part (15). We know that E₃=15V, so lets find the current.
I = V/R = E₃/R₃ = 15/5 = 3A
Since current is same throughout the circuit, I₁=3A as well. Use this to calculate the voltage through R₁.
V = IR = I₁R₁ = (3)(2) = 6V
