The planet Mercury is roughly two-fifth of the size of Earth. Write the fraction as a decimal.
Our job here is to take the number two-fifths and write it as a decimal number. What we know about place value is the number to the left of the decimal is ones place and the number to the right of the decimal is tenths place.
The fraction two-fifths does not have a whole number. This means there’s a zero in the ones place. Now we need to figure out how to write two-fifths as a tenth. The tenth decimal place can be written as a fraction out of 10. Is there a way to take two-fifths and turn it into a fraction out of 10?
We know that five times two equals 10. We also know that if you multiply the denominator by something, you have to multiply the numerator by the same amount. Two times two equals four. Two-fifths equals four 10ths. And four 10ths can be written in the tenths place. Two-fifths written as a decimal equals four 10ths, 0.4.