a and b are correct
for left and right hand sums the error bound is
En =[ K(b-a)^2]/2n
where K is the max of the first derivative and n is the number of sub intervals
13(1.5)^2/(2n) < .1
solving this inequality we get n = 147 ( to the nearest integer)
which means he would have to take readings about every .6 min or every 36 secs.