$2.19 is the rate of change
Something is strange in the wording of the question. "Of the division problems, 4/9 of the problems he got incorrect. What fraction of the division problems did Gavin get incorrect?" :) Well 4/9 of course. One statement answers another. Maybe they wanted the fraction of those wrong answers were in relation to the entire test???

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What exactly do u want us to do?
The small right angled triangle with hypotenuse vale 5 is similar to the big triangle with b as the hypotenuse
Corresponding sides are in the same ratio so we can write
a/12 = 5/b.....(1)
Also applying pythagoras theorem to the large triangle
b^2 = 5^2 + 12^2
b^2 = 169
b = 13
So substituting in equation (1):-
a / 12 = 5 / 13
a = (5*12 ) / 13
Simple interest formula = Principal * Rate * Time / 100
250*7.2*7/100
12,600/100
= 126