Question

Riley adds 45.3 and 3.21 should his sum be greater than or less than 48 tell how you know

Answer 1

From it saying Riley ADDS you know its subtracting, so add 45.3 and 3.21 together. 
Which is 48.51, so it is greater then 48.

Answer 2

His sum should be more than 48.

I know because if you throw away the decimal pieces
and just add up the whole numbers  45  and  3 ,
right there you would have 48 .  So when you include
the decimal pieces, they'll make it even bigger.