Mark has 1/5 as many pencils as Tom. Mark has 20 fewer pencils than Tom. How many pencils does Mark have?
Assuming Tom has x pencils
Mark's pencils = 0.2x
Mark's pencil's = x - 20
0.2x = x - 20 (Mark has the same number of pencils as himself)
20 = 0.8x (Add 20 to the left side, subtract 0.2x from the right side)
x = 25 (Divide x by 0.8)
Mark has 25 pencils