For a resistor at constant temperature, the current is directly proportional to the voltage across it:
V = I × R
Rearranged: I = V / R and R = V / I.
Memory triangle: put V on top and I and R at the bottom. Cover the quantity you want to find.
Example 1.2. A 9 V battery is connected across a 450 Ω resistor. Find the current. I = V / R = 9 / 450 = 0.02 A = 20 mA.
Example 1.3 (LED resistor, used in every Arduino lab). An LED needs about 20 mA and drops about 2 V. The supply is 5 V. Find the series resistor. Voltage across resistor = 5 − 2 = 3 V. R = V / I = 3 / 0.02 = 150 Ω. 150 Ω is itself a standard value, but in the lab we usually take the next common value up, 220 Ω, which keeps the current lower and safer (about 13.6 mA).