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You can draw a Venn diagram. 3 Intersecting circles!

Say Milk alone buyers = a;
Chicken alone buyers = b;
Apple alone buyers = c;

Milk & Chicken buyers = x;
Milk & Apple buyers = y;
Apple & Chicken buyers = z;

All 3 buyers = 10%; Given.

As total no. of buyers are 100% then, we can write...

100 = a+b+c+x+y+z+10; => (a+b+c)+(x+y+z) = 90 ..........(1)

Again, we can write...

a+x+y+10 = 60 => a+x+y = 50..........(2)
b+x+z+10 = 50 => b+x+z = 40..........(3)
c+z+y+10 = 35 => c+z+y = 25..........(4)

Adding eqn. 2,3, & 4 we get...(a+b+c)+2.(x+y+z) = 115 ..........(5)

By subtracting Eqn. 5 from Eqn 1, we get...

x+y+z = 25. Ans D.

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Shalabh Jain,
e-GMAT Instructor

Basically, if someone is in two groups they're counted twice, so we need to subtract them once; if someone is counted in three groups they're counted three times, so we need to subtract them twice.

So:

Total = total group a + total group b + total group c - (ab + ac + bc) - 2(abc)

100 = 60 + 50 + 35 - (doubles) - 2(triples)

100 = 145 - 2(10) - doubles
doubles = 145 - 20 - 100
doubles= 145 - 120 = 25

Note that if there were also some people in none of the 3 groups, the formula would have been:

Total = total group a + total group b + total group c - (ab + ac + bc) - 2(abc) + total in none of a/b/c

but in this question we know that every shopper buys at least one product.