First isolate y by moving the constant over to the other side of the equation. Then you want to divide by the value of y, to get just y.
20 + 20y = 640 ⇔ subtract 20 from both sides to isolate y
20 -20 +20y = 640 - 20
20y = 620
20y/20 = 620/20
y= 31
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