21. Gill is now 27 and has moved into a new flat. She has four pictures to hang in a horizontal row on a wall which is $4800 \mathrm{~mm}$ wide. The pictures are identical in size and are $420 \mathrm{~mm}$ wide. Gill hangs the first two pictures so that one is on the extreme left of the wall and one is on the extreme right of the wall. She wants to hang the remaining two pictures so that all four pictures are equally spaced. How far should Gill place the centre of each of the two remaining pictures from a vertical line down the centre of the wall?
A $210 \mathrm{~mm}$
B $520 \mathrm{~mm}$
C $730 \mathrm{~mm}$
D $840 \mathrm{~mm}$
E $1040 \mathrm{~mm}$
21. C The diagram shows part of the wall of width $4800 \mathrm{~mm}$ and the four equally spaced pictures, each of width $420 \mathrm{~mm}$. Let $x$ be the required distance, that is the distance from the centre of each of the two pictures in the middle of the wall to a vertical line down the centre of the wall (marked by a broken line). Then the distance between the centres of any two adjacent pictures is $2 x$. Note that the distance between the centres of the two pictures on the extremes of the wall is $(4800-2 \times 210) \mathrm{mm}=4380 \mathrm{~mm}$. Therefore $2 x+x+x+2 x=4380$.
So $x=4380 \div 6=730$. Hence the required distance is $730 \mathrm{~mm}$.