Joshua spent 3/7 of his money on souvenirs, so he is left with 4/7 of his money. He has $16 left, so his initial money was:
$16/ (4/7)
= $16* (7/4)
= $28.
3/7 of his money (or the amount of money he spent on souvenirs) was:
$28* (3/7)= $12.
Joshua spent $12 on souvenirs.
Hope this helps~