All integer solutions (let $x$ = number of $10 tickets$, $y$ = number of $20 tickets$) are:
- For $y=0,1,\dots,8$, any integer $x$ with $20-y \le x \le 28-2y$.
There are 45 possible combinations. The maximum number of tickets is 28 (all $10$-dollar tickets); the maximum number of $20$-dollar tickets is 8 (with $x=12$).
Explanation
- Write the constraints:
- At least 20 tickets: $x+y \ge 20$.
- Cost no more than \$280: $10x+20y \le 280$.
- $x,y$ are nonnegative integers.
- Simplify the cost inequality by dividing by 10:
- $x+2y \le 28$.
- Combine inequalities to bound $y$:
- Subtract $(x+y \ge 20)$ from $(x+2y \le 28)$ to get $y \le 8$.
- So $y=0,1,\dots,8$.
- For each fixed integer $y$ in that range, solve for $x$:
- From $x+y \ge 20$ we get $x \ge 20-y$.
- From $x+2y \le 28$ we get $x \le 28-2y$.
- So $20-y \le x \le 28-2y$ (integers).
- Count solutions:
- For $y=0$ there are $9$ values ($x=20\ldots28$),
for $y=1$ there are $8$ values, …, for $y=8$ there is $1$ value.
- Total $9+8+\dots+1 = 45$ combinations.
Examples of extremes:
- Maximum tickets: $x=28,y=0$ gives 28 tickets costing \$280.
- Maximum $20$-dollar tickets: $y=8,x=12$ gives 20 tickets costing \$280.
- Minimum cost while meeting at least 20 tickets: $x=20,y=0$ costing \$200.