**Solution —**

∵ 5 dolls = 6 balls

∴ 1 dolls = 6 / 5 balls

∴ 10 dolls = 6/5 ×10 = 12 balls

∴ 10 dolls + 18 balls = 12 balls + 18 balls

= 30 balls

∴ The cost of 6 balls = Rs 15

∴ The cost of 1 ball = Rs 15 / 6

∴ The cost of 30 balls = Rs.15 / 6 × 30

**= Rs. 75**

Thus, Cost of 10 dolls and 18 balls = Rs.75