we can write out: 0.15x+0.14y=5 or 15x+14y=500
1. 15x and 500 is divisible by 5, so 14y has to be also divisible by 5
2. It is possible for y=0,5,10,15.... or y=5k where k in an integer.
3. 15x+14*5k=500 --> 3x=100-14k
4. find the least number k that 100-14k is divisible by 3: 100 is not, 86 is not, 72 - is divisible (k=2) and therefore, x=10.
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