"The total number of dollars in fives and tens" tells you that these denominations are being added together. Because a five dollar bill is worth $5 and a ten dollar bill is worth $10, then the number of $5 bills is represented by 5f and a the number of $10 bills is represented by 10t. Therefore,
Scientific notation is a way of expressing numbers that are too large or too small to be conveniently written in decimal form. It may be referred to as scientific form or standard index form, or standard form in the UK.
