Question

27 x 15 = (20 x 15)-(7 × 15).
45 x 18 = (40 × 10) + (40 × 8) + (5 × 10) + (5 × 8).
99 x 31 = (100 x 31) – (1 × 31) _
64 x 15 = 32 x 30