Why do Helios and Boreas agree to have the
contest?

A dispute of power.

Explanation:

Helios and Boreas agreed to a competition to prove who was the most powerful between the two. Boreas, The Northern Wind, tried to blow the man's clothing off with all of his might. The more he tried, the tighter and more snug the garment became. Boreas called to Helios to prove his strength. Helios, The Bright Sun, shone his warm rays of light upon the man, and slowly he took off his cloak.

I love Greek mythology, so I hope this helps! :)

onto a large boat, and mark the water level that the boat sinks to. then, take the elephant out and put a bunch of marked weights onto the boat until it reaches the same water level. add up the masses of the weights to get the weight of the elephant.

because they were not just any seashells they were sally's seashells