4. Solve the problems: 7 * 4 3 * 9 2 * 3 24 : 6 21 : 7 27 : 9 81 - 40 + 9 56 + 30 - 7 67 - 7 - 4 51 - 15 : 5 51 - 21 : 3 51 - 24 : 4

Let's solve these problems step-by-step:

1. Multiplication:
   • 7 * 4 = 28
   • 3 * 9 = 27
   • 2 * 3 = 6
2. Division:
   • 24 : 6 = 4
   • 21 : 7 = 3
   • 27 : 9 = 3
3. Mixed operations:
   • 81 - 40 + 9 = 41 + 9 = 50
   • 56 + 30 - 7 = 86 - 7 = 79
   • 67 - 7 - 4 = 60 - 4 = 56
4. Mixed operations with division:
   • 51 - 15 : 5 = 51 - 3 = 48
   • 51 - 21 : 3 = 51 - 7 = 44
   • 51 - 24 : 4 = 51 - 6 = 45

Answers:

• 7 * 4 = 28
• 3 * 9 = 27
• 2 * 3 = 6
• 24 : 6 = 4
• 21 : 7 = 3
• 27 : 9 = 3
• 81 - 40 + 9 = 50
• 56 + 30 - 7 = 79
• 67 - 7 - 4 = 56
• 51 - 15 : 5 = 48
• 51 - 21 : 3 = 44
• 51 - 24 : 4 = 45