1/3 * 420 = amount of t-shirts sold in the morning
1/3 * 420 = 420/3 = 140 t-shirts
140 t-shirts were sold, so remaining is 420 - 140
420 - 140 = 280 remaining t-shirts
4/7 * 280 = amount of t-shirts sold in the afternoon
4/7 * 280 = (280/7) * 4 = 40 * 4 = 160 t-shirts
160 t-shirts were sold, so remaining is 280 - 160
the remaining t-shirts = 120 t-shirts
120 t-shirts are left to sell at the end of the day
