GMAT Maths Arithmetic - Operations with Numbers

Ratios
A ratio is a comparison of two or more numbers with the same unit label.A ratio can be written in three ways:
a: b
a to b
or

a/b

A rate is similar to a ratio except that the unit labels are different. For example, the expression 25 miles
per hour is a rate — 25 miles/1 hour.

Proportion
Two ratios set equal to each other is called a proportion. To solve a proportion, cross-multiply.

2/3 = 2/x

2x = 6

therfore x = 3

Percentage

Percentage is a way of expressing a number as a fraction of 100 (per cent meaning "per hundred"). It is often denoted using the percent sign, "%". For example, 45% (read as "forty-five percent") is equal to 45 / 100, or 0.45.

Percentages are used to express how large one quantity is, relative to another quantity. The first quantity usually represents a part of, or a change in, the second quantity, which should be greater than zero. For example, an increase of $ 0.15 on a price of $ 2.50 is an increase by a fraction of 0.15 / 2.50 = 0.06. Expressed as a percentage, this is therefore a 6% increase.

Simple Interest

Interest = Principal (amount invested) × Interest rate (as a decimal) × Time (years) or I = PRT.

Exponents
The exponent of a number tells how many times to use that number as a factor. For example, in the expression
43, 4 is the base number and 3 is the exponent, or power. Four should be used as a factor three times: 43
= 4 × 4 × 4 = 64.