If he spent 3/7 of his money on souvenirs, it means that he had kept 4/7 of his money...
4/7 * M = 16
7 * 4/7 * M = 16 * 7
4M = 112
M = 112/4
Therefore:
M = 28
--------------
Answer:
Joshua had $28 to begin with but spent $12 of his money.
We can check if this is the correct result by multiplying $28 by 3/7.
3/7 * $28
= ($28*3)/7
= ($84)/7
= $12