You may not have seen what the ordinate is,

The ordinate is the frequency/group distance,

Not the frequency,

So if you want to know the frequency of each segment, you have to multiply it by the group distance.

The group spacing is 2.5,

The frequency/group interval of not less than 22.5 hours is 0. 16, 0.08 and 0.04.

Add them up and multiply them by the group distance of 2.5.

You will know the frequency of no less than 22.5 hours.

Which is 0.7,

Multiply the total number of people by 0.7 to get the result.