Olympiad Maths Prep

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Problem 985

AIME late
Algebra Difficulty 6.0 Find the answer

Galochkina A.I.

Sasha's collection consists of coins and stickers, and there are fewer coins than stickers, but there is at least one. Sasha chose some positive number \ \mathrm{t}>1 \ (not necessarily an integer). If he increases the number of coins by $\$ \mathrm{t} \ times, without changing the number of stickers, then his collection will have \ 100 \ items. If instead he increases the number of stickers by $\$ t \ times, without changing the number of coins, then he will have \ 101 \ items. How many stickers could Sasha have? Find all possible answers and prove that there are no others.

Official solution

Let's denote the number of coins by \ \mathrm{~m} \ and the number of stickers by $\$ \mathrm{n} \, then the condition can be rewritten as a system of equations \ \ begin $\{$ cases $\} \mathrm{mt}+\mathrm{n}=100, \ \mathrm{~m}+\mathrm{nt}=101$. \end } \{ cases \} \$ \$ Subtract the first equation from the second: \$ $1=101-100=(\mathrm{m}+\mathrm{nt})-(\mathrm{mt}+\mathrm{n})=(\mathrm{n}-\mathrm{m})(\mathrm{t}-1) . \$ \ Therefore, \ \mathrm{t}=1+(\mathrm{frac}\{1\}\{\mathrm{n}-\mathrm{m}\} \. Now add the two initial equations: \$ $201=101+100=(\mathrm{mt}+\mathrm{n})+(\mathrm{m}+\mathrm{nt})=(\mathrm{m}+\mathrm{n})(\mathrm{t}+1) . \$ \ Therefore, \ \mathrm{t}=\midfrac frac \{201\}\{\mathrm{m}+\mathrm{n}\}-1 \. Introduce the notations $\$ a=n-m \$, \$ b=n+m \. Note that \a>0 \, since \ n>m \. Equate the two expressions for \$t\$: $\$ \$ 1+\mid$ frac $\{1\}\{n-m\}=\backslash$ frac $\{201\}\{m+n\}-1 \backslash \backslash$ Leftrightarrow $\backslash 1+\mid$ frac $\{1\}\{a\}=\mid$ frac $\{201\}\{b\}-1 \backslash \backslash$ Leftrightarrow $\backslash$ frac $\{2 a+1\}\{a\}=\mid f r a c\{201\}\{b\} . \$ \ Note that the numbers \ 2 a+1 \ and \$a are coprime, so the fraction $\ frac {2a+1}\{2 a+1\}

!
\ 67and and \ 201 \.Since. Since \ 2 a+1>1 \$, we need to consider three cases.

1) \ 2 a+1=3 \. Then $\$ a=1 \ and \frac frac \{201\}\{b\}=\mid f r a c\{3\}\{1\} \.3 .3 so, \ b=67 \, from which $\$ \$ m=\mid f r a c\{1\}\{2\}(b-a)=33$, \quad $\mathrm{n}=\mid \operatorname{frac}\{1\}\{2\}(\mathrm{a}+\mathrm{b})=34 . \$ \ Also, \ \mathrm{t}=2 \. It is easy to verify that this case works. 2) $\$ 2 \mathrm{a}+1=67 \. Then \ \mathrm{a}=33 \ and $\$|\operatorname{frac}\{201\}\{\mathrm{b}\}=|$ frac $\{67\}\{33\} \. So, \ \mathrm{~b}=99 \, from which $\$ \$ \mathrm{~m}=\mid$ frac $\{1\}\{2\}(\mathrm{b}-\mathrm{a})=33$, Iquad $\mathrm{n}=\mid$ frac $\{1\}\{2\}(\mathrm{a}+\mathrm{b})=66 . \$ \ Also, \ \mathrm{t}=\midfrac frac \{34\}\{33\} \. It is easy to verify that this case works. 3) $\$ 2 a+1=201 \. Then \ a=100 \ and $\ frac \{201\}\{b\}=\mid f r a c\{201\}\{100\} \.So,. So, \ b=100 \,fromwhich, from which \ \ m=\mid f r a c\{1\}\{2\}(b-a)=0,quadn=frac a)=0, |quad n=|frac \{1\}\{2\}(a+b)=100 . \ \$ This case does not work, as there must be at least one coin.

## Answer

34 or 66 stickers.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.