Work backwards from the 84 marbles Ali had at the end.<br><br>
[[model:questions/solutions/q10429.png]]<br><br>
After giving to David, 84 marbles remained. This is 70% of the amount before giving to David (since 30% was given away).<br>
&bull; Before giving to David = 84 ÷ 70 × 100 = 84 ÷ 0.7 = 120<br>
&bull; Before losing 15 marbles: 120 + 15 = 135<br>
&bull; 135 is 75% of the remainder after Ben (since 25% was given to Chloe).<br>
&bull; Remainder after Ben = 135 ÷ 75 × 100 = 135 ÷ 0.75 = 180<br>
&bull; 180 is 80% of the original marbles (since 20% was given to Ben).<br>
&bull; Original marbles = 180 ÷ 80 × 100 = 180 ÷ 0.8 = 225<br><br>
Ali had 225 marbles at first.<br>
Check: 20% of 225 = 45 to Ben; 225 − 45 = 180; 25% of 180 = 45 to Chloe; 180 − 45 = 135; lost 15; 135 − 15 = 120; 30% of 120 = 36 to David; 120 − 36 = 84. ✓