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MATH / How many paths are there from A to B on the lines of the grid shown, if every step must be up or to the right?[asy]size(3cm,3cm);int w=6;int h=3;int i;for (i=0; i<h; ++i) draw((0,i…

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Answer published by the source. Consult the official source to check your work against its answer.

problem

How many paths are there from AA to BB on the lines of the grid shown, if every step must be up or to the right?[asy]size(3cm,3cm);int w=6;int h=3;int i;for (i=0; i<h; ++i){ draw((0,i) -- (w-1,i));}for (i=0; i<w; ++i){ draw((i, 0)--(i,h-1));}label("BB", (w-1,h-1), NE);label("AA", (0,0), SW);[/asy] For example, here is one such valid path highlighted in blue: [asy] size(3cm,3cm); int w=6; int h=3; int i; for (i=0; i<h; ++i){ draw((0,i) -- (w-1,i)); }for (i=0; i<w; ++i){ draw((i, 0)--(i,h-1)); } label("BB", (w-1,h-1), NE); label("AA", (0,0), SW); draw((0,0)--(1,0), blue+1.5); draw((1,0)--(1,1), blue+1.5); draw((1,1)--(2,1), blue+1.5); draw((2,1)--(3,1), blue+1.5); draw((3,1)--(4,1), blue+1.5); draw((4,1)--(4,2), blue+1.5); draw((4,2)--(5,2), blue+1.5); [/asy]
Plain-text mathematical notation (without MathML)
How many paths are there from A to B on the lines of the grid shown, if every step must be up or to the right?[asy]size(3cm,3cm);int w=6;int h=3;int i;for (i=0; i<h; ++i){ draw((0,i) -- (w-1,i));}for (i=0; i<w; ++i){ draw((i, 0)--(i,h-1));}label("B", (w-1,h-1), NE);label("A", (0,0), SW);[/asy]
For example, here is one such valid path highlighted in blue:
[asy]
size(3cm,3cm);
int w=6;
int h=3;
int i;
for (i=0; i<h; ++i){ 
draw((0,i) -- (w-1,i));
}for (i=0; i<w; ++i){ 
draw((i, 0)--(i,h-1));
}
label("B", (w-1,h-1), NE);
label("A", (0,0), SW);

draw((0,0)--(1,0), blue+1.5);
draw((1,0)--(1,1), blue+1.5);
draw((1,1)--(2,1), blue+1.5);
draw((2,1)--(3,1), blue+1.5);
draw((3,1)--(4,1), blue+1.5);
draw((4,1)--(4,2), blue+1.5);
draw((4,2)--(5,2), blue+1.5);
[/asy]
Original LaTeX notation
How many paths are there from $A$ to $B$ on the lines of the grid shown, if every step must be up or to the right?[asy]size(3cm,3cm);int w=6;int h=3;int i;for (i=0; i<h; ++i){ draw((0,i) -- (w-1,i));}for (i=0; i<w; ++i){ draw((i, 0)--(i,h-1));}label("$B$", (w-1,h-1), NE);label("$A$", (0,0), SW);[/asy]
For example, here is one such valid path highlighted in blue:
[asy]
size(3cm,3cm);
int w=6;
int h=3;
int i;
for (i=0; i<h; ++i){ 
draw((0,i) -- (w-1,i));
}for (i=0; i<w; ++i){ 
draw((i, 0)--(i,h-1));
}
label("$B$", (w-1,h-1), NE);
label("$A$", (0,0), SW);

draw((0,0)--(1,0), blue+1.5);
draw((1,0)--(1,1), blue+1.5);
draw((1,1)--(2,1), blue+1.5);
draw((2,1)--(3,1), blue+1.5);
draw((3,1)--(4,1), blue+1.5);
draw((4,1)--(4,2), blue+1.5);
draw((4,2)--(5,2), blue+1.5);
[/asy]

level

Level 3

type

Counting & Probability

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Official source

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