Answer:
A = 26
Explanation:
sum of students = classA + classB + classC
let's say classA = A, classB = B, and classC = C
A + B + C = 66
class A has five more students than class B, so A = 5 more than B so A = 5+B
class C has 2 less students than class B, so C = 2 less than class B = B -2, so C = B-2
A + B + C = 66
A = 5+B
C = B-2
substitute 5+B for A and B-2 for C in the first equation to limit this to one variable (B)
(5+B) + B + (B-2) = 66
3B + 3 = 66
subtract 3 from both sides to isolate the variable and its coefficient
3B = 63
divide both sides by 3 to solve for B
B = 21
A = 5 + B = 5 + 21 = 26