& \mathbf{Describe:} && \text{Two scales}\\&&& A: \ \text{2 yellow blocks weigh 12 pounds}\\&&& B: \ \text{1 yellow block and 1 red block weigh 15 pounds}\\& \mathbf{My \ Job:} && \text{Figure out the weight of one yellow block and one red block.}\\& \mathbf{Plan:} && \text{Start with} \ A. \ \text{Figure out the weight of one yellow block.}\\&&& \text{In} \ B, \ \text{replace the yellow block with its weight.}\\&&& \text{Figure out the weight of the red block.}\\& \mathbf{Solve:} && A: \ \text{yellow block + yellow block  = 12 pounds, so one yellow block is half of 12, or 6 pounds.}\\&&& B: \ \text{Replace yellow block with 6.}\\&&& \text{Then 6 + red block = 15 pounds.}\\&&& \text{Red block = 15 - 6, or 9 pounds.}\\& \mathbf{Check:} && \text{Replace each yellow block with 6 pounds.}\\&&& \text{Replace the red block with 9 pounds.}\\&&& \text{Check with the scale weights.}\\&&& A: \ \text{6 + 6 = 12 pounds} \quad B: \ \text{6 + 9 = 15 pounds}