Goal Seek changes one input to hit one target. Solver changes many inputs to get the best result while respecting limits.

Turn it on

File → Options → Add-ins → Manage Excel Add-ins → Go → tick Solver Add-in. It appears under the Data tab.

Example: product mix

You make three products. Each uses machine hours and labour hours, both limited. How many of each maximises profit?

• B2:D2 — quantities (the cells Solver will change; start them at 0).
• B3:D3 — profit per unit. Total profit in F3: =SUMPRODUCT(B2:D2,B3:D3).
• Machine hours used in F4: =SUMPRODUCT(B2:D2,B4:D4), limit in G4.
• Labour hours used in F5: =SUMPRODUCT(B2:D2,B5:D5), limit in G5.

Solver settings

• Set Objective: $F$3, To: Max.
• By Changing: $B$2:$D$2.
• Constraints: $F$4 <= $G$4, $F$5 <= $G$5, $B$2:$D$2 = integer, $B$2:$D$2 >= 0.
• Method: Simplex LP (the model is linear).

Click Solve → Keep Solver Solution. Ask for the Answer report to see which limits are “binding” — those are the bottlenecks worth expanding.