There's no magic method to doing these, and problem 21 makes it harder to see a pattern by using inputs that are not consecutive integers (like problem 20 does)!
Looking at inputs 20 and 21, the outputs are 8 less than the inputs. Check the other inputs and notice that the same pattern holds up.
-25+4y=35 *Subtract 35* -25+4y-35=0 *add 25 (to cancel out)* 4y-35=25 *add 35 (to cancel out)* 4y=60 *Divide by 4 to get y on its own* y=15 Hope this helps :)
