Problem
The left pan has 5 green marbles, each 24 g. Add them up (multiply) to get the total weight the right pan must match.
Five equal groups of 24 g is just repeated addition, which a learner can do as 24+24+24+24+24 or as one multiplication.
4.NBT.B.5Identify SubproblemsOn the right pan, the 3 yellow marbles are known: each is 28 g. Add them to see how much weight is already accounted for.
Splitting the right pan into 'yellow part' and 'red part' turns one hard pan into two easy pieces.
4.NBT.B.5Identify SubproblemsThe level scale forces the right pan to 120 g too, so the two red marbles carry 120 - 84 = 36 g.
A level scale is a picture of an equal sign: whatever the left side weighs, the right side must weigh too, so the leftover after the yellows is exactly what the reds supply.
1.OA.D.7Draw A DiagramThe two red marbles together weigh 36 g.
The right pan holds only two kinds of marble, the 3 yellow and the 2 red, so taking the yellow weight out of that pan's total leaves exactly the red weight.
That pan's total has to be 120 g, because a level scale means both pans hold the same weight and the left pan's 5 green marbles come to 120 g.
Five marbles that each weigh the same 24 g are five equal groups of 24, and counting equal groups is what multiplying does.
Two equal reds share that 36 g, and 2 x 18 = 36 with 84 + 36 = 120, so one red is 18 g.
Finding the number that doubles to 36 is the same as splitting 36 into two equal parts; testing 18 confirms the whole scale balances.
3.OA.A.4Guess And CheckA level scale is a giant equal sign: make both pans weigh the same and the missing weight pops right out.