Her weekly allowance is $80
She needs $400 (699.99-299.99) to buy the camera.
Because it is a 5 week period before she buys the camera, you divide $400 by 5 = $80
To check that is right you:
$80 x 5 = $400. She already has $299.99 and if you add $400 and $299.99 = $699.99