So, to begin, we can set the scenario into an equation to solve for the amount of magnets that Sarah had...
Joseph had 28 magnets, plus an unknown amount, x :
28 + x
Sarah had 157, plus the same unknown amount.
157 + x
Also, Sarah had four times the amount of Joseph at the end:
4(28 + x)
As a full equation:

Now, all we have to do is solve for x, the unknown amount:

Use the Distributive Property on 4(28 + x):

Subtract x on both sides:


Subtract 112 on both sides:


Divide by 3 on both sides:

15 is the unknown amount added to each pile of magnets!
Hope I could help you out!
Have a good one, and God bless!