21/42 = 1/2

One way is to find a number (or numbers) that divide into both numbers with an even (whole number) quotient. (A common divisor) The other way is to simply multiply both the nominator and the denominator by a whole number (other than 1).

Since photosynthesis is the conversion of carbon dioxide and water into a sugar called glucose using sunlight energy,water is the key for this reaction to take place