Question 4768
Let x = the number of widgets that John sold
Let 2.5x= the number of widgets Harvey sold (2.5 times as many as John)

the difference indicates a subtraction problem:
2.5x-x=87
Combine like terms:
1.5x=87
Divide both sides by 1.5:
x=58

John sold 58 units, Harvey sold 145 units (2.5*58)

145-58=87 so this is correct.