1.8/X=0.625 can be solved by removing the decimals. To do this, you need to multiply by a power of 10 that will remove them. Count the number of decimal places on the number that has most decimal places. In this case it's 0.625 with 3 decimal places. The power of 10 is 3, which is 1 followed by 3 zeroes: 1000. To multiply by 1000 all you do is move the decimal place to the right by 3: 1800/X=625. You add zeroes when you run out of decimal places. So 1.8 becomes 1.800. Now multiply both sides by X: 1800=625X or 625X=1800, same thing. Now divide through by 625: X=1800/625. This fraction cancels down by dividing top and bottom by 25 (or divide by 5 twice=360/125)=72/25=2 22/25.
To get the fraction part back into a decimal: 22/25=(4*12)/(4*25)=88/100=0.88 making the answer 2.88. Another way to convert the fraction into decimal is to look at the number of digits in the denominator and multiply by the power of 10 that has as many zeroes. So 25 has 2 digits and 100 has 2 zeroes, so the decimal will have 2 decimal places: 22/25*100 cancels to 88, and putting in 2 decimal places this gives us 0.88. So 2 22/25=2.88 decimal.