Work backwards from the 280 shirts left at the end.<br><br>
[[model:questions/solutions/q10428.png]]<br><br>
After Tuesday's sales, 280 shirts remained. This is 80% of the total before Tuesday's sales (since 20% was sold).<br>
&bull; Total before Tuesday sales = 280 ÷ 80 × 100 = 280 ÷ 0.8 = 350<br>
&bull; 350 is after 50 shirts arrived on Tuesday. So before arrival = 350 − 50 = 300<br>
&bull; 300 is 60% of the original stock (since 40% was sold on Monday).<br>
&bull; Original stock = 300 ÷ 60 × 100 = 300 ÷ 0.6 = 500<br><br>
The shop had 500 shirts at first.<br>
Check: 40% of 500 = 200 sold Mon; 500 − 200 = 300; 300 + 50 = 350; 20% of 350 = 70 sold Tue; 350 − 70 = 280. ✓