well, if you cannot think of the general of the sum, then use common sense. since the nth number is quite less, we can substitute the value one by one.
for example, for first question, it will be
90(1/1 + 1/2+ 1/3+ 1/4+ 1/5)
Of course your answer is wrong. . eg. should be obvious.
Just write each sum out in full and then add up using basic arithmetic.
There is certainly no formula for the sum of reciprocals. The fact that there are only a few terms in each of your sums should be the clue that you write the sum out in full and then calculate.