“number of $50 notes is 45% of number of $10 notes” 

“she spent 20% of $50 notes”, so the number of $50 notes reduced to 36% of number of $10 notes. (45*80%=36)

“As a result, she had 192 more $10 notes than $50 notes”, meaning 64% of $10 notes is 192. (1-36%=64%)

therefore, 1% = 192/64=3, 100% =300. She had 300 $10 notes.

The rest is easy.