So, first turn your fractions into decimals. ⅞ = 0.875 which rounded is 0.88. 4/5 = 0.8. Next, put the decimals into the equation to replace the fractions. 0.88 + 4.2 - 0.8. Next solve the problem. So 0.88 + 4.2 = 5.08. Now your equation should look like this 5.08 - 0.8. Then subtract 0.8 from 5.08 which gives you 4.28. So 4.28 is your answer.