Kay bought 3 watches and 20 rings for $3250. A watch costs $125 more than a ring. Find the cost of a ring....

Let's use English.

Lets Watch = x ,, Ring = y
A watch cost 125 more than a ring 
so X = y + 125. Make sense right ?

3 Watches + 20 rings =3250
3 x + 20 y = 3250

Remember that x = y + 125 

3(y+125) + 20 y = 3250
3y + 375 + 20y = 3250
23y = 2875
y = 125, So the cost of a single ring is $125

Hope that answers your question :)