To find out the profit at the same rate, we need to calculate the profit as a ratio to the cost of the shirts.
Profit ratio = Profit / Cost of shirts.
Given that the trader made a profit of 3.50 naira on shirts he bought for 28 naira,
Profit ratio = 3.50 / 28 = 0.125
To calculate the profit the trader would make on goods that he bought for 42 naira, we multiply the profit ratio by the cost of the new goods.
Profit = Profit ratio * Cost of new goods
Profit = 0.125 * 42 = <<0.125*42=5.25>>5.25
Therefore, the trader would make a profit of 5.25 naira on goods he bought for 42 naira.