Lesson 5.13, one day (day 133). Thirteen slides. All prices marked [update].
Do now
- A pair of jeans costs $60 [update].
- You wear them 200 times before they wear out.
- How much did each wearing cost you?
- Show the division.
Teacher notes
two minutes. Take the answer and the division out loud. $60 ÷ 200 = $0.30 per wear. Insist on the unit. Then the hook: a $15 shirt worn five times. Do that in your head and tell me which one was expensive.
Cost per wear
- Cost per wear = price ÷ the number of times worn
- The answer is in dollars per wear.
- Price is what you pay once.
- Cost per wear is what the item actually costs you.
Worked problem (a), the jeans
- Price: $60 [update]
- Wears: 200
- $60 ÷ 200 = $0.30 per wear
Worked problem (b), the shirt
- Price: $15 [update]
- Wears: 5
- $15 ÷ 5 = $3.00 per wear
- Cheapest price on the slide. Most expensive to own.
Worked problem (c), the coat
- Price: $90 [update]
- Worn 3 winters. How many wears is that?
- How many times do you wear a winter coat in one winter? ______
- 3 × ______ = ______ wears
- $90 ÷ ______ = $______ per wear
- The estimate is the argument. You have to decide the number of wears, and then defend it.
Teacher notes
let two students disagree about how many cold days there are. That disagreement is the standard (SUS 1. f), the cost and benefit of a decision). At 60 wears per winter the answer is $0.50 per wear.
What makes a garment last, 1: fiber
- Cotton: comfortable, absorbent, wrinkles, shrinks
- Wool: warm even when damp, can felt with heat and agitation
- Polyester: resists wrinkles, dries fast, holds odor, melts under a hot iron
- Spandex or elastane: stretch and recovery, and it is the fiber that fails first
- A cotton and polyester blend survives washing better than either one alone.
Teacher notes
Image: four fabric swatches labeled with their fiber.
What makes a garment last, 2: seam type
- Single-needle: one row of stitching. Opens first.
- Double-needle: two parallel rows. Holds.
- Flat felled: raw edges folded and wrapped inside, then stitched through. Strongest.
- Serged edge: thread looped over a raw edge. That is a finish, not a seam.
Teacher notes
turn a pair of jeans inside out and find the flat felled side seam. Then find the serged edge inside a cheap shirt.
Image: four close-ups, one per seam type, from the inside of a garment.
What makes a garment last, 3: stitches per inch
- Put the ruler on a structural seam.
- Count the stitches in one inch.
- 8 or fewer is low.
- 10 to 12 holds.
- More stitches per inch means more thread holding the same inch of seam.
Teacher notes
Image: a ruler along a seam with a bracket marking one inch.
What makes a garment last, 4: reinforcement at stress points
- A stress point is anywhere the body pulls on the garment.
- Pocket corners. Belt loops. Under the arm. The knee. Where a strap joins.
- Look for a bar tack: a short, dense line of stitches at that spot.
- A gusset, a doubled layer, a knee patch: all reinforcement.
- No reinforcement at a stress point is where the garment will fail.
Teacher notes
Image: a bar tack at the corner of a jeans pocket, magnified.
What makes a garment last, 5: buttons and closures
- Four to six passes of thread through the holes.
- A shank: a little slack so the button sits above the fabric and the buttonhole has room.
- A knot on the wrong side that holds, with a tail, not cut flush.
- A zipper that runs smoothly both ways.
Teacher notes
hold up a well-sewn button and a loose one. Do not sew today.
Image: a button with a visible shank next to a button lying flat with two loose threads.
Inspection checklist, five checks per garment
| Check | Good |
|---|---|
| Fiber from the label | Percentages printed, a blend or a durable fiber |
| Seam type | Double-needle or flat felled on load-bearing seams |
| Stitches per inch | 10 to 12 |
| Reinforcement at a stress point | A bar tack, a gusset, or a doubled layer |
| Buttons and closures | Enough passes, a shank, a knot that holds |
Teacher notes
three minutes per station, one bell, three rotations. Refuse to say whether a garment is "good." The checklist decides.
The trade-off
- Garment 2, the thin t-shirt: $8 [update]
- Garment 1, the heavy sweatshirt: $44 [update]
- Person 1: walks to school, wears it four days a week for two winters.
- Person 2: needs a shirt for one concert.
- Compute cost per wear for both garments, for both people.
- Then say who should buy which.
Teacher notes
the point out loud: cost per wear only answers the question once you know who is wearing it and how often.
Exit card
- 3 things you would check on a garment before buying it
- 2 cost-per-wear numbers you computed today, with units
- 1 item in your own closet you now think was a good buy or a bad buy, and why in six words
Teacher notes
three students read. The "needs more" pile decides whether Lesson 5.14 Day 2 opens with a 90-second cost-per-wear review before the graded task.