Find unknown weights on mobiles and scales: both sides weigh the same, and taking equal weights off both sides keeps the balance.
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The top number is the total; a level bar means both sides weigh the same.
The number in the circle at the top is the total weight hanging below it.
Using 'half the total on each side' to find hidden weights.
Problem. Find the unknown weights in Figs. 7.1 and 7.2. (Textbook p. 164)
The book's Figs. 7.3 to 7.8, ready for the questions below.
Mobiles A, B and C (Textbook pp. 164–165)
Find the hidden weights in the book's mobiles.
Mobile A (total 8): what do the book and one pencil box weigh?
Taking equal weights off both plates keeps the scale balanced.
A level scale means both plates weigh the same.
If you take the same weight off both plates, the scale stays level.
Finding a sack's weight by removing matching items.
Problem. Find the weight of the sack. In Fig. 7.10 all the sacks weigh the same. (Textbook p. 165)
Figs. 7.11 and 7.12, for the questions below.
Scale G (Fig. 7.11, Textbook p. 166). All sacks weigh the same.