Answer:
Sally's team found 50 balls
Explanation:
300 balls are hidden.
300x30%x40%
=36 balls are yellow
Sally's team finds 20% of 36 and 80% of the white balls (300x30%-36).
20%x36+80%x(300x30%-36)
=7 (approximately or else they would've found 7.2 yellow balls) +80%x(90-36)
=7+43 (approximately)
=50