There are 20 sandcastles on Mark's beach, each with 10 towers. On Jeff's beach, there are three times as many castles as on Mark's beach, each with 5 towers. What is the combined total number of sandcastles and towers on Mark's and Jeff's beaches?
A step-by-step solution is:
580
The total number of towers at Mark's beach is 20 * 10 = 200 towers.

There are 200 + 20 = 220 sandcastles and towers on Mark's beach.

Jeff's beach has 3 * 20 = 60 sandcastles, three times as many as the number of sandcastles in Mark's beach.

The number of towers in each of the sandcastles on Jeff's beach is 1 / 2 * 10 = 5.

Since each sandcastle has 5 towers on Jeff's beach, there are 5 * 60 = 300 towers.

In total, on Jeff's beach, there are 300 + 60 = 360 towers and sandcastles.

Altogether, 360 + 220 = 580 sandcastles and towers can be found on their beaches.

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So, the final answer is 580