Set x equal to the amount of money Kim gets, and y for Molly's money.
x = 2/5y, since the ratio is 2:5.
y = 105 + x, since Molly has 105 more pounds than Kim.
Substitute x into this equation, so that y = 105 + 2/5y. Subtract 2/5y from both sides to get 3/5y = 105. Divide each side by 0.6 (equivalent to 3/5) and you get y = 175, which is how much Molly has. Kim gets 2/5 of Molly's share (or 105 fewer pounds), so she has 70.
70 + 175 (Kim and Molly's shares) is equal to 245. Therefore, Laura receives the remaining 140 pounds.
140/385 = a/100
Cross multiply so that 385a is 14000. Divide by 385 to get 36.4 (rounded).
Thus, Laura receives about 36.4% of the total money. Hope this helps (: