Work backwards from the 39 kg left after all usage.<br><br>
[[model:questions/solutions/q10422.png]]<br><br>
After baking bread, 39 kg remained. This is 65% of the remainder after cakes (since 35% of that remainder was used for bread).<br>
&bull; Remainder after cakes = 39 ÷ 65 × 100 = 39 ÷ 0.65 = 60 kg<br>
&bull; Mrs Lee started with 120 kg, so flour used for cakes = 120 − 60 = 60 kg<br><br>
Mrs Lee used 60 kg of flour for cakes.<br>
Check: 120 − 60 = 60; 35% of 60 = 21 kg for bread; 60 − 21 = 39 kg left. ✓