If 3/5 of all the brownies is 21, then 5/3 of 21 is the number of the brownies that Lauren brought in.
That probably sounded super confusing. When multiplying a number by a reciprocal of a fraction (a fraction where the numerator and denominator is reversed), the previous fraction multiplication is reversed.
Multiplying 21 with 5/3 will give us the original number of brownies.
Lauren originally brought 35 brownies.