Maths Olympiad Prep

Library / /8 of 18

Algebra Difficulty 4.4 AIME Find the answer United States

Each of 27 bricks (right rectangular prisms) has dimensions a×b×ca \times b \times c, where aa, bb, and cc are pairwise relatively prime positive integers. These bricks are arranged to form a 3×3×33 \times 3 \times 3 block, as shown on the left below. A 28th brick with the same dimensions is introduced, and these bricks are reconfigured into a 2×2×72 \times 2 \times 7 block, shown on the right. The new block is 1 unit taller, 1 unit wider, and 1 unit deeper than the old one. What is a+b+ca + b + c?
Figure 1
Figure 2

Pick one

Solution

Without loss of generality, assume a<b<ca < b < c. Comparing the figures and considering the change in orientation gives rise to the equations 3a+1=2b3a + 1 = 2b, 3b+1=2c3b + 1 = 2c, and 3c+1=7a3c + 1 = 7a. To solve this system of linear equations, use the first two equations to write aa and cc in terms of bb, namely a=23b13a = \frac{2}{3}b - \frac{1}{3} and c=32b+12c = \frac{3}{2}b + \frac{1}{2}. Substituting these into the third equation gives 92b+32+1=143b73\frac{9}{2}b + \frac{3}{2} + 1 = \frac{14}{3}b - \frac{7}{3}. Multiplying both sides by 6 yields 27b+9+6=28b1427b + 9 + 6 = 28b - 14, which shows that b=29b = 29. Back substituting then gives a=19a = 19 and c=44c = 44. The requested sum is 19+29+44=9219 + 29 + 44 = 92.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.