AI Tutor
Imagine you have a ribbon that is 22 meters long, and you cut off 4545 centimeters. If you just subtract 2−452 - 45, you get −43-43 meters of ribbon!
That negative answer sounds silly because the numbers are speaking two different measurement languages. You can never add, subtract, or compare numbers until they share the same unit of measurement.
How do you decide which unit to change before you start solving? Let's break down the rule.

The Golden Rule: Convert First

Before you do any arithmetic, scan every single number in the word problem to check its unit. A common unit is a single, matching measurement unit that all values are converted into before calculating.
To pick the best unit, check the question first: if it asks for 'answers in grams,' convert everything to grams. If no unit is requested, converting to the smaller unit usually prevents messy decimals.
What happens when a single problem throws three different units at you all at once?

Length: Combining Three Different Units

A carpenter joins three wood boards together: Board A is 1.2 m1.2\text{ m}, Board B is 85 cm85\text{ cm}, and Board C is 450 mm450\text{ mm}. What is the total length of the bench in centimeters?
Since the question asks for centimeters, convert Board A by multiplying by 100100 (1.2 m=120 cm1.2\text{ m} = 120\text{ cm}) and Board C by dividing by 1010 (450 mm=45 cm450\text{ mm} = 45\text{ cm}). Now that all three are in centimeters, add them up: 120+85+45=250 cm120 + 85 + 45 = 250\text{ cm}.