## Solve Lesson 13 Page 39 Math Study Topic 10 – Kite>

Topic

Uncle Thanh wanted to buy a gift for his son on the occasion of his birthday, so he went to a toy store. I plan to choose 1 out of 5 toys. In the store, each toy only has 10 different products for sale. Know that if you buy a remote control helicopter kit, you will only buy 1 product; if you buy a lego set, you will buy 3 different products; if you buy a solar powered robot kit, you will buy 5 different products; if you buy rubik, you will buy 7 different products; If you buy a dinosaur model, you will buy 9 different products. How many ways does Uncle Thanh buy birthday gifts for his children?

Detailed explanation

+ Case 1: If Uncle Thanh buys a remote control helicopter, he will only buy a remote control helicopter => There is 1 way to choose

+ Case 2: If Uncle Thanh buys a lego set, he will buy 3 different products => Choose 3 products out of 10 other products of the same type => Yes \(C_{10}^3\) how to choose

+ Case 3: If Uncle Thanh buys a solar-powered robot assembler, he will buy 5 different products => Choose 5 products out of 10 other products of the same type => Yes \(C_{10} ^5\) how to choose

+ Case 4: If Uncle Thanh buys rubik, he will buy 7 different products => Choose 7 products out of 10 other products of the same type => Yes \(C_{10}^7\) how to choose

+ Case 5: If Uncle Thanh buys a dinosaur model, he will buy 9 different products => Choose 9 products out of 10 other products of the same type => Yes \(C_{10}^9\) how to choose

=> Number of ways to choose is: \(1 + C_{10}^3 + C_{10}^5 + C_{10}^7 + C_{10}^9\)