Final answer:
0.151 is the smaller decimal number when compared to .251, as the digits after the decimal point determine that 1 is less than 2.
Step-by-step explanation:
When comparing two decimal numbers to determine which decimal number is smaller, 0.151 or .251, we look at the values digit by digit from left to right. Both 0.151 and .251 have a 0 before the decimal point, so we compare the numbers after the decimal point. The first digit after the decimal in 0.151 is 1, while for .251 it is 2. Since 1 is less than 2, we can conclude that 0.151 is the smaller number. It is important to understand that when comparing decimal numbers, all digits are significant, and a leading zero before the decimal point, like in 0.151, is there to help us see the decimal point more clearly.