The total number of towers at Mark's beach is 20*10 = <<20*10=200>>200 towers.
There are 200+20 = <<200+20=220>>220 sandcastles and towers on Mark's beach.
Jeff's beach has 3*20 = <<3*20=60>>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 = <<1/2*10=5>>5
Since each sandcastle has 5 towers on Jeff's beach, there are 5*60 = <<5*60=300>>300 towers.
In total, on Jeff's beach, there are 300+60 = <<300+60=360>>360 towers and sandcastles.
Altogether, 360+220 = <<360+220=580>>580 sandcastles and towers can be found on their beaches.
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