Step 1:

A has scored 20 marks more than B: $A = B + 20$ B has scored 5% less than A: $B = A - 5\%$ of $A$ $B = A - 0.05A = 0.95A$

Step 2:

From $A = B + 20$, substitute $B = 0.95A$ into this: $A = 0.95A + 20$

Step 3:

$A - 0.95A = 20$ $0.05A = 20$ $A = 20 / 0.05 = 400$

Step 4:

Since $B = 0.95A$: $B = 0.95 \times 400 = 380$