To find out how many blocks to shade in a 10 by 10 grid to represent 13%, you first calculate the total number of blocks in the grid.
A 10 by 10 grid has: \[ 10 \times 10 = 100 \text{ blocks} \]
Next, to find 13% of 100 blocks, you can use the following calculation: \[ \frac{13}{100} \times 100 = 13 \text{ blocks} \]
Therefore, to represent 13% in a 10 by 10 grid, you would shade 13 blocks.