Anurse worked from 8: 15 am until 11: 30 am and 12: 30 am until 4: 15 pm. estimate the total number of hours worked

Ok so lets work it out ( answer at the bottom in bold) 

11:30 - 8 :15 = 3:15 ( 3 hours and 1 5 minutes) 
and for the next part its a little trickky cause we are going from am to pm but we can work it out
so we know it will end in 45 because it takes 30 + 30 = 1 hour and it says 4: 15 so we know we need a 15 and 15 + 30= 45 
so now we will put 45 to the side but we now instead of having 12:30 we have 1 :00 pm and 4:00 pm
4:00 - 1 : 00 = 3 pm and now we take the 45 and add it to the 3:00 pm so 3:00 + 45 = 3:45 pm
and now we add our 2 end results 
3 :45  + 3:15 = 7 hours
The nurse worked 7 hours

Technically speaking, histograms are another version of bar charts, if you need to work one out then skim through google hope-o that -o!