The first answer is 18. The process is like this, because it was originally a two-digit number, and later it was a single digit with a decimal point and a mantissa. Then, the sum after the decimal point of the sum of these two numbers 19.8 is the original single digit of this number. So you can confirm the single digit tail of this two-digit number. Now if the sum is 19.8, it means that the 10 bit of this two-digit number is 1. Then we know from this that this two-digit number is a number with a decimal number of 1 and a unit number of 8, which is 18. Then do your own calculation according to the topic, 18 plus 1.8= 19.8. It means that you have calculated correctly. This two-digit number is 18.