Amos spent 1/4 of his money and had 3/4 of his money left.

His sister spent 7/10 of her money and had 3/10 of her money left.

At the end, Amos had twice as much as his Sister.

Let the Sister’s amount of money in the end be 3 units. Then Amos had 6 units in the end.

Since 3/10 of the sister’s money represents 3 units, the sister had 10 units of money at first.

Since 3/4 of Amos’ money represents 6 units, Amos had 8 units at first.

They had a total of 18 units of money at first.

18 units = $1674
1 unit = $1674/18 = $93
6 units = $93 x 6 = $558

So Amos had $558 in the end.

3 units = $93 x 3 = $279

So his Sister has $279 in the end.

They had a total of $558 + $279 = $837 in the end.

Compare this with the $1674 starting amount of money.

Decrease in amount of money
= $1674 - $837
= $837

Percentage decrease
= $837/$1674 x 100%
= 50%