If Rin's snorkel helmet has 75 craters, then Dan's and Daniel's helmets combined have 75-15 = <<75-15=60>>60 craters.
Let's assume Dan's helmet has C craters.
Since Dan's skateboarding helmet has ten more craters than Daniel's ski helmet, the total number of craters their helmets have is C+(C-10)=60
2C=60+10
The total number of craters that Dan's helmet has is C=70/2
Dan's skateboarding helmet has 35 craters.
#### 35