Hi Julia,

I think you son is probably learning the place value system. In a two digit number the leftmost digit tells you the number of tens and the rightmost digit the number of ones or units. Thus if you have three tens and five ones then the number you have is thirty five or 35. Tens are like dimes and ones like pennies so if he has five dimes and two pennies he has 52 cents.

I hope this helps,
Harley