An abacus is a counting frame and has been used extensively throughout Asia and Africa to help with calculating. It is an excellent resource for helping your child understand 2-digit numbers up to 99. These maths worksheets show clearly the two columns, the units and the tens. In other words the 2 yellow balls in the tens column have a value of 20 whilst the two balls in the units column have a value of 2.
This will also help later when multiplying by 10 as they can see each digit moving one place to the left. You can pick up an abacus from shops such as The Early Learning Centre but they tend to have the rows of beads going across rather than vertically, which is not as helpful.
Remember, most of our sets of worksheets have at least 4 pages on the topic shown and full answers.