Problem
Three equal sides of 0.58 m: the wire used is 0.58 x 3 = 1.74 m.
Adding three equal decimals is the same as multiplying by 3; lining up the hundredths keeps the place values straight.
5.NBT.B.7Identify SubproblemsThe wire bent into the equilateral triangle uses 1.74 m, which is the triangle's perimeter, three times its 0.58 m side.
With no overlapping, the wire lies exactly once along all three sides, so the wire used equals the perimeter, the sum of the three equal sides.
The three sides join end to end with no gaps and no double-backs, so their lengths add up to the whole length of wire laid along them.
Equilateral means all three sides are the same 0.58 m, so three equal sides are three equal groups of 0.58 m, which is 3 times 0.58.
Three groups of 0.58 come to 1.74 when the place values stay lined up: three groups of 58 hundredths make 174 hundredths, and ten hundredths bundle into a tenth and ten tenths into a whole, giving 1.74.
The leftover is the total minus the wire used: 4 - 1.74 = 2.26 m.
Think of 4 as 4.00; subtracting 1.74 from 4.00 gives 2.26 by regrouping, just like subtracting whole-number money amounts.
5.NBT.B.7Identify SubproblemsAdd up the three equal sides, then subtract from the total: leftover wire is just a tidy decimal subtraction you already know!