Answer:
$180
Step-by-step explanation:
The problem asks us to find the maximum value that Weiming must pay. Weiming is paying more than Joyce, and the total of their payments will be the cost of the present. Hence both will be paying the maximum amount when the present's cost is at its maximum: $210. If w represents the amount Weiming pays in that case, then the amount Joyce pays is 210-w.
The problem statement tells us two things about what Weiming pays. It sets a minimum of 2 times what Joyce pays, and it sets a maximum of 150 more than what Joyce pays. The problem requests the <em>maximum</em> Weiming pays, so we're only interested in the limit on the maximum.
w ≤ 150 +(210 -w) . . . . . Weiming pays at most 150 more than Joyce pays
2w ≤ 360 . . . . . add w, collect terms
w ≤ 180 . . . . . divide by 2
Weiming pays at most $180.
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<em>Additional comment</em>
If we consider the more general case where the cost of the present is "c", then Joyce pays (c-w) and the relations become ...
2(c -w) ≤ w ≤ 150 +(c -w) . . . . Weiming pays between twice Joyce's cost and 150 more than Joyce's cost
2c -2w ≤ w ≤ 150 +c -w . . . . . eliminate parentheses
This is better solved by writing it as two inequalities:
2c -2w ≤ w ⇒ 2c ≤ 3w ⇒ 2/3c ≤ w
w ≤ 150 +c -w ⇒ 2w ≤ 150 +c ⇒ w ≤ 75 +c/2
That is, the amount Weiming pays will be ...
2/3c ≤ w ≤ 75 +c/2 . . . . . . between 2/3 the present cost and $75 more than half the present cost.
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Some compound inequalities can be solved by manipulating the entire expression. Here, we cannot isolate w to the middle expression when we keep the compound inequality together, so we must divide it into its parts to solve it.