Risa used
16 of the thread to tie some presents. She threw away
310 of the remaining thread. As a result, she had 21 cm of the thread left. How much thread did Risa have at first? Give your answer in cm.
Fraction of the thread left after tying some presents
= 1 -
16=
56 Fraction of the remaining thread left after throwing away
= 1 -
310=
710
Fraction of the total thread left
=
56 ×
710=
712
Length of the thread at first = 12 u
Length of the thread left = 7 u
7 u = 21
1 u = 21 ÷ 7 = 3
Length of the thread at first
= 12 u
= 12 x 3
= 36 cm
Answer(s): 36 cm