3·x = 12                   x = Cloze (1):JXUwMDZj

3·R = 12                  R = Cloze (2):JXUwMDZj

3·Q = 12                 Q = Cloze (3):JXUwMDZj

When the unknown quantity is in the denominator, we will take an intermediate step. If 100 divided into x gives 4, 4 multiplied by x makes 100:

100 = 4 · x

And now we solve as before.

100/x = 4                 x = Cloze (4):JXUwMDZhJXUwMDA3

100 / I = 4                I = Cloze (5):JXUwMDZhJXUwMDA3

100 / r ² = 4               r ² = Cloze (6):JXUwMDZhJXUwMDA3

If the unknown quantity is squared you must take its square root and, therefore, take it on the other side as well:

r ² = 25                         r = Cloze (7):JXUwMDZk

Now do the same with letters that represent numbers instead of numbers:

V = I·R                            I = Cloze (8):JXUwMDBl Cloze (9):JXUwMDc3 Cloze (10):JXUwMDBh

v = e / t                            e = Cloze (11):JXUwMDJl Cloze (12):JXUwMGVm Cloze (13):JXUwMDJj

R = ρ · L / A                     L = R Cloze (14):JXUwMGVm A Cloze (15):JXUwMDc3  ρ

R = ρ · L / A                     A = ρ · Cloze (16):JXUwMDE0 / Cloze (17):JXUwMDBh

F = k · q · Q / r²                        r² = k · q · Q Cloze (18):JXUwMDc3 F                        r = √ (k · q · Q Cloze (19):JXUwMDc3 F)

Audio: 

1. Calculate the resistance of an iron knowing that a current of 11.5 A flows through it when it is plugged into the domestic mains ( V = 230 V ). R = Cloze (1):JXUwMDZhJXUwMDAy Ω

2. Calculate the current that would pass through a hair dryer with a resistance of 23 Ω when plugged into the domestic mains ( V = 230 V ). I =  Cloze (2):JXUwMDY5JXUwMDAxJXUwMDEwJXUwMDYx

3. If a 4 Ω bulb carries a current of 3 A when connected to a battery, what is the electromotive force (emf) provided by the battery? (Remember that the battery's emf is the same as the voltage consumed by the light bulb) V = Cloze (3):JXUwMDY5JXUwMDAzJXUwMDEyJXUwMDc2