For this question it's best to draw a picture.

Home----(2 blocks)----stoplight, 8:20am ----(x distance)---- town library, 8:24-----(3 blocks)---- school

so you know 3 blocks is halfway. The total distance must be 6 blocks.

2 blocks from home is the stoplight, x distance must be one block since that's the remainder of half the distance.

He walked x distance in 4 minutes (8:24 minus 8:20) which means he walks one block in 4 minutes.

4 minutes for each block, three blocks to go, 4 times 3 is 12. 12 minutes after 8:24 (his halfway point time) is 8:36.

Also, assuming the same pace, he must have left home at 8:12.

Home----(2 blocks)----stoplight, 8:20am ----(x distance)---- town library, 8:24-----(3 blocks)---- school

so you know 3 blocks is halfway. The total distance must be 6 blocks.

2 blocks from home is the stoplight, x distance must be one block since that's the remainder of half the distance.

He walked x distance in 4 minutes (8:24 minus 8:20) which means he walks one block in 4 minutes.

4 minutes for each block, three blocks to go, 4 times 3 is 12. 12 minutes after 8:24 (his halfway point time) is 8:36.

Also, assuming the same pace, he must have left home at 8:12.