8/25 of the toffee is left because you need to add how much Dawn ate and how much Elijah ate. Therefore you would find a common denominator for the two fractions which is 25 and you would multiply 5 by both the numerator and denominator of 2/5 and add it to 7/25. That is the fraction that is eaten. Then you would take the whole which is 1 or 25/25 minus the fraction eaten which is 17/25
7/25+10/25= 17/25
25/25 - 17/25= 8/25
