Answer:
Step-by-step explanation:
Depending on the tools available to your 11y.o., it's a typical question for linear inequalities.
Where a,b,c are the # of 1,15,50 coins given by the buyer, and d,e,f the # of coins given back by the seller.
1×a + 15×b + 50×c > 1×d + 15×e + 50×f
a+b+c+1>=d+e+f
a+b+c >=2
Now, there are other clues in the text :
If the seller gives back money, then you didn't pay with 1's, otherwise, it means you overpay and you could have given fewer coins. So a=0.
Similarly, the seller cannot give you back a coin of 50, otherwise, you could have given fewer coins as the buyer. So f=0.
So that gives :
15×b + 50×c > 1×d + 15×e
b+c+1>=d+e
b+c >=2
From that point on, there is only a handful of scenarios to test.
