The average marks scored by three students on a test were 55. When two more students (A and B) were added, the new average marks were reduced by 5. If B scored 15 marks more than A, what were the marks scored by A?

35

Sum of marks of three students = 55 x 3 =165

Sum of marks when A and B were added = 50 x 5 = 250

Sum of marks of A and B = 250 - 165 = 85

Lets assume marks of A is x

Given marks of B = x + 15

⇒ x + x + 15 = 85

⇒ x = 35

Marks scored by A is 35.