Here is some help on a couple of your problems.
Draw a diagram.
Let x mph be the speed of the southbound train then the speed of the northbound train is x + 20 mph. In the first 4 hours after noon the northbound train goes 4(x + 20) miles. In the next hour it goes a further x + 20 miles and the southbound train goes x miles. From the diagram the distance between the trains at 5:00 pm is (x + 20) + 4(x + 20) + x miles. Set this equal to 160 miles and solve for x.
Suppose that the sum of Clark's scores on the 15 weekly tests is x. If the weight of each of the 15 weekly tests is 1 and the average is 91 then x/15 = 91. Thus x = 15(91) = 1365. If his score on the final exam is F, its weight is 5 and the overall weighted average is the sum of the weighted scores divided by the sum of the weights. That is (1365 + 5F)/(15 + 5). But this is 85 so solve (1365 + 5F)/(15 + 5) = 85 for F.Cheers,