Children should be very familiar with counting up and down in fives before attempting these worksheets. All the numbers on the pages are multiples of 5. Join them in pairs so that each pair adds up to 100. Watch out for a common error with this type of work whereby children add an extra 10 to the answers, thinking that, for example,  65 + 45 = 100 as they do not take account of the two fives making another whole ten. It's not as easy as it looks to keep track of the pairs, even when drawing lines. it could be suggested that they use a pack of crayons to colour pairs the same colour. Pairs that make 100