8.454 is 3 tenths more than 8.154
3.625 is 2 and 2 tenths more than 1.425
Place values after the decimal point are tenths, hundredths, thousandths, ten thousandths...so on.
So 3 tenths looks like this 0.30 ⇒ 3 is in the tenths place. When you say 3 tenths more, it is a clue that the operation to be done is addition. thus,
8.154
+ 0.30
8.454 * in addition or subtraction, the decimal points must be aligned to avoid confusion.
2 and 2 tenths is like this 2.20 ⇒ 1st 2 is in the ones or units place and the 2nd 2 is in the tenths place. Again it states more than so addition must be done.
1.425
+ 2.20
3.625