①
2/5 of the remainder was used for cookies.

1 - 2/5 = 3/5

So 3/5 of the remainder was the rest.

②
Now 2/7 of the rest was used for pizza dough.

1 - 2/7 = 5/7

So 5/7 of the rest was left over and stored inside the container.

③
5/7 of the rest = 5/7 x 3/5 of the remainder
= 3/7 of the remainder

So, stored flour = 3/7 of the remainder.

④
Now, question says amount used to bake the cupcakes was twice the amount stored in the container.

Amount used to bake cupcakes is therefore

= 2 x 3/7 of the remainder
= 6/7 of the remainder

So amount used to bake cupcakes
= 6/13 of total

(From the fraction 6/7, we can tell that the remainder was 7 units and the amount used to bake cupcakes was 6 units.

Total amount of flour = 13 units

Furthermore, we can conclude that the amount stored in the container is 3 units)

So, 5.4kg = 13 units

3 units = 5.4kg x 3/13

≈ 1.24615385 kg

≈ 1.246 kg

= 1kg 246g (nearest gram)

Thank you for the detailed working and explanation. They help in my understanding.