120 people in a Thrissur ward were surveyed: 10 use none of three apps and 5 use all three. The note gives each app's total. How many use exactly two apps (the three ? parts together)?

Question

Accepted Answer

15. At least one = 120 - 10 = 110. The totals add to 50 + 45 + 40 = 135, which counts exactly-two people twice and all-three people three times: 135 = 110 + E + 2 × 5, so E = 15. 25 forgets the 2 × 5, 20 takes away the centre only once, and 10 is the number using none.

120 people in a Thrissur ward were surveyed: 10 use none of three apps and 5 use all three. The note gives each app's total. How many use exactly two apps (the three ? parts together)?

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