Q: What is the solution?
Solve -171 = -40*s + 21*s for s.

A: 9

Q: What is the solution?
Solve -185*i + 165*i - 420 = 0 for i.

A: -21

Q: What is the solution?
Solve 15*u = 16*u - 1 for u.

A:
1