Question

A construction worker is pouring concrete stairs. The first step requires 1.7 cubic feet of concrete, and the first 4 steps require a total of 17 cubic feet. If the steps follow an arithmetic series, how much concrete is required for the first 12 steps

Answer 1

Based on the given parameters of a = 1.7 cubic feet and concrete for first 4 steps as 17 cubic feet, the concrete is required for the first 12 steps is 132.6 cubic feet

Arithmetic progression

• First term, a = 1.7 cubic feet
• Sum of first four terms = 17 cubic feet

Sn = n/2 {2a + (n - 1) d}

17 = 4/2{2×1.7 + (4 - 1)d}

17 = 2{3.4 + (3)d}

17 = 2(3.4 + 3d)

17 = 6.8 + 6d

17 - 6.8 = 6d

10.2 = 6d

d = 10.2/6

• Common difference, d = 1.7

Concrete required for first 12 steps;

Sn = n/2 {2a + (n - 1) d}

= 12/2{2×1.7 + (12-1)1.7}

= 6{3.4 + (11) 1.7}

= 6(3.4 + 18.7)

= 6(22.1)

= 132.6

Concrete required for first 12 steps = 132.6 cubic feet

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