For the first equation all unknown numbers were substituted by a variable and the author then added. For the second equation the author multiplied values in each variable and added them together. Similarities between the two are illustrations by tables help visualize, each gives variables full quantity and value, and each can be solved by elimination or substitution. The difference between the two, are utilization and system equation methods.
You go to the bank and get change for $100 in $1 bills and $10 bills and there are 55 bills total. How many $1 bills and how many $10 bills.