1. How did you do that, I really don't see where the -10, 13 and 15 came from unless im being really stupid :|

2. Originally Posted by ramdrop
How did you do that, I really don't see where the -10, 13 and 15 came from unless im being really stupid :|
That was a hypothetical example; I meant for you to use it as a model to solve the actual problem. It was so that I could show you how to do it without actually solving the problem for you.

3. Oh right okay, well I solved it to have:

$-4 \le x \le 7$

Seeing as -4 is impossible the smallest value must be 0 and the maxium, 7

4. Originally Posted by ramdrop
Oh right okay, well I solved it to have:

$-4 \le x \le 7$

Seeing as -4 is impossible the smallest value must be 0 and the maxium, 7
You seem to have kept the loosest bounds and discarded the strictest.

$\displaystyle x \ge 0$

$7-x \ge 0 \implies x \le 7$

$6-x \ge 0 \implies x \le 6$

$4+x \ge 0 \implies x \ge -4$

Discarding the second and fourth, I get $0 \le x \le 6$ .

IMPORTANT: x is not the value you seek to find the min and max of. You want to find min and max of "the number of male patients under 50 without a back problem" which is 7-x. So...

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