In the table, we have...
35 red shirts
55 blue shirts
15 green shirts
65 black shirts
30 white shirts
In total, there are 35+55+15+65+30 = 200 shirts in all.
The proportion of red shirts is equal to the number of red shirts over the total, so
proportion of red shirts = (number of red shirts)/(total)
proportion of red shirts = 35/200
proportion of red shirts = 0.175
So 17.5% of the shirts are red. This means that the manager would expect the same proportion or percentage no matter what the new total turns out to be.
Take 17.5% of the new total 1800 to get...
17.5% of 1800 = 0.175*1800 = 315
The manager should order 315 new red shirts