He got $250 from his father

And the mother got 250$

It's like 500$

He bought the cell phone for $450

Harty paid 50$, zhpka paid 30$

it was 20$

His father gave him $10 and his mother gave him $10

Dad owes you $240 and Mom owes you $240

It adds up to 240 \$+240\$+30\$=510\$

The question is where did the 10 dollars go