Question: What is the solution?

Solve -4*r + r + 31*r + 420 = 0 for r.

Answer: -15

Question: What is the solution?

Solve -f + 21 - 19 = 0 for f.

Answer: 2

Question: What is the solution?

Solve 530*h + 25 = 535*h for h.

Answer: 5

Question: What is the solution?

Solve -3377*s = -3357*s + 140 for s.

Answer:
-7