P4P5 Math: Before and After Start Same End Multiples

One of the "Before and After" problem sums where the 2 parties start with equal value.
After each undergo a decrease, one of value remaining is a multiple of the other.

Such questions are common in P4 and P5 exams.
They either appear in Paper 1 of P5 or Section C of P4 papers.

Calculator usage is not allowed.

Evelyn and Kathy each have the same number of stamps at first.
After Evelyn sold 24 stamps and Kathy gave away 100 stamps,
Evelyn now has thrice as many stamps remaining as Kathy.

How many stamps did Evelyn have at first?


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