This may be too old fashioned (but OldTrout was chosen to reflect my great age as much as anything else)....
With both little fish and this year with small fry on tricky numerical questions (so not simple calculations but word problems or data problems where there's a lot of information to sort through my advice has been this:
1. Read through the initial information (the info text/ the graph/ the table).
2. Read through what it is they want to find out. It sometimes helps if you underline the key thing you need to solve - maybe How much will tickets cost (when there are a range of people going) or average height of boys (from a table with various bits of data on height for boys and girls).
3. Then go back to text/ table/ graph and find the information that can help you solve that problem.
4. Ask yourself if the information is there in a form to use for the answer or if you have to do something with it (say add up all the boy heights and then divide by # of boys).
5. Then work out the answer.
6. Then make sure of format. Sometime you just need to select or write down the number and sometimes you have to include mm or convert from metres to centimetres.
Three generally good things to drill in now:
1) Write out your calculations - often mistakes are made because you are calculating in your head. If it's on paper you can see it and if there is time you can even check your maths.
2) If there is any conversion involved for the answer (i.e. they want the answer in m but your data is in mm, cm and metres but they want the answer in metres) - convert data first before any calculations.
3) Often tables or graphs will have odd intervals for data - so a graph may have the y axist increasing by intervals of 10 but the x axis increasing by intervals of 50 - make sure you take the time to really understand how the data is presented. A classic trick is to have symbols for counts (say a star for 6 children) and for the child to just count each star as one rather than six.
Hope that helps.
Invaluable advice! I am printing it out, thank you!