Let us assume the total amount of money Joshua had = x
Amount of money Joshua had left after spending on souvenir = $16
Then
(3x/7) + 16 = x
3x + 112 = 7x
112 = 7x - 3x
112 = 4x
x = 112/4
= 28
Now
The amount of money spent on souvenir = (3/7) * 28
= 3 * 4 dollars
= 12 dollars
So the total amount of money spent on souvenirs by Joshua is $12