Any two units have a special conversion factor for converting between them. If you don’t know what it is, you can look it up online or in a textbook.
Remember, multiplying a number by 1 doesn’t change its value. That means you can multiply your measurement by this factor and the length will be the same. The only difference is the units: your answer will be written in inches instead of millimeters. [3] X Research source
16 mm x 1in25. 4mm{\displaystyle {\frac {1in}{25. 4mm}}} = ?
16 mm x 1in25. 4mm{\displaystyle {\frac {1in}{25. 4mm}}} = ? Cancel out the millimeters:16 x 1in25. 4{\displaystyle {\frac {1in}{25. 4}}} = ? Since the only unit left is inches, your answer will be in inches.
16 x 1in25. 4{\displaystyle {\frac {1in}{25. 4}}} = 16 in ÷ 25. 4 = about 0. 63 inches
1 cm = about 0. 4 inches 1 cm = 10 mm 1 inch = exactly 2. 54 cm (you don’t need this for this conversion, but it’s a good one to know)
430 mm= 430 mm x 1cm10mm{\displaystyle {\frac {1cm}{10mm}}}= 43 cm.
Divide by two, then subtract 1/10 of the cm value: 43 cm ÷ 2 = 21. 5→ 1/10 of 43 is 4. 3→ subtract the second number from the first: 21. 5 - 4. 3 = 17. 2 in. Or divide by ten, then multiply by four:43 cm ÷ 10 = 4. 3→ 4. 3 x 4 = (4 x 4) + (0. 3 x 4) = 16 + 1. 2 = 17. 2 in.
In the example, the exact answer is actually 16. 921 inches. The estimate is less than a third of an inch off, which is pretty good for mental math.
