Answer: R = 394.36ohm
Explanation: In a LR circuit, voltage for a resistor in function of time is given by:

ε is emf
L is indutance of inductor
R is resistance of resistor
After 4s, emf = 0.8*19, so:





R = 394.36
In this LR circuit, the resistance of the resistor is 394.36ohms.
On Earth, a cannonball with a mass of 20 kg would weigh 196 Newtons.
With the formula F=mg, where F is the weight in Newtons, m is the mass, and g is the acceleration due to gravity on the Earth which is 9.8m/s^2.
F=20kg x 9.8m/s^2= 196 Newtons
BUT on the moon, acceleration due to gravity is 1.6 m/s^2,
so F=mg=20kgx1.6m/s^2= 32 N
I would assume that the magnesium would have to come into contact with an ion in order to become a magnesium ion.
You look up the element in the periodic table. Subtract the atomic number (small number) from the mass number (big number). The answer is the number of neutrons.